Well and formation tests, which entail taking measurements while flowing fluids from the reservoir, are conducted at all stages in the life of oil and gas fields, from exploration through development, production and injection. Operators perform these tests to determine whether a formation will produce, or continue to produce, hydrocarbons at a rate that gives a reasonable return on further investments. Operators also use test data to determine the limits of the reservoir and to plan the most efficient methods for producing wells and fields.
During testing, operators measure formation pressure, characterize the formation fluids and reservoir and determine permeability and skin—damage to the formation incurred during drilling or other well operations. Data that indicate how the formation reacts to pressure increases and decreases during a test can also reveal critical information about the reservoir.
Well and formation tests are also primary sources of critical data for reservoir models and are the principal means by which engineers confirm or adjust reservoir model parameters. Engineers use these models to understand how reservoir fluids, the formation and the well interact and use that knowledge to optimize completion and development strategies.
Operators assess the production potential of wells through several test methods, singularly or in combination. They may choose to perform a production well test in which the well is flowed through a temporary completion to a test separator (Figure 1). Or they may use a wireline formation tester to capture fluid samples and measure pressure downhole at the zone of interest. Engineers sometimes perform both types of tests.
Figure 1. Test separator. Separators are designed so that produced fluids enter the vessel, where they are retained long enough for the oil to separate and float to the top of the water. This process is enhanced by deflector plates that slow flow velocity and by coalescing plates that gather oil into large droplets. Once the oil and water have separated, the oil then flows over a weir into a separate section of the vessel while water remains in the original compartment. Mechanical water- and oil-level controller arms, with attached floats lifted by the rising fluid, trigger valves (not shown) that release oil and water along their respective flowlines. When the fluids reach prescribed levels, the controllers cause the release of gas or air pressure with actuation of pneumatic valves. Mist extractors remove oil droplets from the gas phase before gas exits through a valve at the top of the vessel and passes through an orifice plate meter (not shown) for measurement. Safety valves allow gas to escape into the atmosphere rather than overpressure the vessel.
During production well tests, technicians flow reservoir fluids to the surface through a drillstring or a drillstem test (DST) string. Packers isolate the zone to be tested while downhole, or surface equipment provides well control. The well is flowed at different rates through a choke valve that can be adjusted to control the flow rate precisely.
Reservoir fluids produced to the surface are sent directly to holding tanks until test operators determine that contaminants such as drilling fluids are eliminated, or at least minimized, from the flow stream. After cleanup, flow is redirected to a test separator where bulk fluids are divided into oil, gas and water, and any debris, such as sand and other material, is removed. The three fluid phases are measured and analyzed separately. Operators may opt to obtain additional reservoir and fluid flow data by simultaneously running production logging tools into the well on wireline. These tools measure the downhole flow rate and fluid composition and can indicate which zones are contributing to the total flow.
During well tests, reservoir fluids are produced to the separator at varying rates according to a predetermined schedule. These tests may take less than two days to evaluate a single well or months to evaluate reservoir extent. Test types include buildup, drawdown, falloff, injection and interference. For most tests, engineers permit a limited amount of fluid to flow from or into a formation. They then close the well and monitor pressures while the formation equilibrates.
Buildup tests are performed by shutting in the well after some period of flow to measure increase in bottomhole pressure (BHP). By contrast, for drawdown tests, engineers open the well after a specified shut-in period to observe BHP decrease. During injection tests and falloff tests, fluid is injected into the formation, and BHP, which increases as a result, is monitored. The well is then shut in and the ensuing decreasing BHP is recorded. Interference tests record the pressure changes in adjacent wells when the test well pressure is changed. The time it takes for changes in the test well to affect pressure at the observation well gives engineers an indication of the size of the reservoir and flow communication within it.
Engineers analyze responses to pressure change schemes using pressure transient analysis, a technique based on the mathematical relationships between flow rate, pressure and time. The information from these analyses helps engineers determine the optimal completion interval, production potential and skin. They can also derive average permeability, degree of permeability heterogeneity and anisotropy, reservoir boundary shape and distance, and initial and average reservoir pressures.
Engineers use specific variations on well buildup and drawdown tests to evaluate gas wells. During a backpressure test, a well is flowed against a specified backpressure until its BHP and surface pressures stabilize—an indication that flow is coming from the outer reaches of the drainage area. An isochronal test is a series of drawdowns and buildups. Pumping rates vary for each drawdown, while subsequent buildups continue until the well reaches its original shut-in pressure. A modified isochronal test—in which drawdown and buildup periods are of equal duration—may also be used.
Based on data from these tests, engineers are able to determine production potential, skin and absolute open flow (AOF)—the theoretical rate at which the well would flow if backpressure on the sandface, or the borehole wall, were zero. Operators use AOF as the basis for calculations to determine the relationship between backpressure settings and flow rates of the well.
Rather than use well tests, operators may opt to evaluate their wells using wireline formation testers that include a quartz pressure gauge and a fluid sampling tool placed across a production interval (Figure 2). During these formation tests, reservoir fluids are pumped or flowed into the wireline formation testerthrough a probe inserted into the formation or between packers set above and below the sampling site.
Figure 2. Wireline formation tester sampling. Pistons are driven from one side of the wireline formation tester to force a packer assembly firmly against the formation to be tested. At its center, the packer includes a probe that is then extended into the formation to withdraw wellbore fluids. Formation fluids (red arrows) flow into the probe and into flow lines. The fluids are pumped into the wellbore until they are sufficiently free of contamination as determined by downhole fluid analysis (green and brown cylinders). Uncontaminated fluids are directed into storage bottles (orange) where the fluids are kept at in situ conditions. Multiple samples can be taken on one trip into the well. When all tests are completed, the samples are brought to the surface and may be sent to laboratories for advanced testing. A quartz pressure gauge measures and records bottomhole pressures.
The reservoir fluids, which may be contaminated with drilling fluid, are first flowed or pumped through flowlines in the tool into the wellbore while the contamination level decreases. Once engineers determine that the formation is delivering minimally contaminated reservoir fluids, they redirect flow to sample chambers within the tool. The chambers are retrieved to the surface and transported to laboratories for analysis.
Scientists also use downhole fluid analysis (DFA) to monitor the sampling process. Using optical spectroscopy, or the recorded light spectrum, engineers identify in real time the composition of fluids as they flow into the tool; this method also reveals critical data about the reservoir without waiting for laboratory tests to be completed. Additionally, the DFA measurements confirm that the sample is uncontaminated and eliminate uncertainties associated with fluid transport and laboratory reconstruction of in situ conditions necessary for fluid analysis. Technicians also use DFA data to identify gas/oil ratios, relative asphaltene content and water fraction in real time.
A variety of well and formation test schemes are performed throughout the stages in the life of a well or field. At the exploration stage, operators may use well tests to simulate production after a well is completed to establish production potential and reserves estimates. In addition, capturing large fluid samples at the surface gives experts an opportunity to perform laboratory measurements on the reservoir fluids.
Well tests at the exploration stage also allow operators to determine if low flow rates are affected by skin or are the result of natural permeability of the reservoir. Armed with the knowledge of either situation, engineers can then take appropriate actions, plan treatments that may be necessary once production commences or decide to abandon the project for economic reasons. For instance, well tests can be used to estimate reservoir size, which allows operators to abandon a small reservoir that will not be economical despite high initial flow rates.
During the field development stage, well tests help indicate wells that may require stimulation treatments. Using well test data, engineers predict induced or natural fracture length and conductivity. They can then estimate productivity gains that may be realized from a stimulation treatment. In addition, wireline formation testers can be used for pressure testing to determine static reservoir pressures and to confirm fluid contacts and density gradients. This information helps analyze communication within the reservoir, tie reservoir characteristics to a geologic model and identify depleted zones.
During the production phase, well tests are aimed at monitoring reservoirs, collecting data for history matching—comparing actual production with predicted production from reservoir simulator—and assessing the need for stimulation. These tests use a pressure gauge placed at formation depth to collect data during pressure buildup and drawdown.
Well productivity usually diminishes over time, sometimes as a result of formation damage from fines migration—the movement of very small particles through the formation to the wellbore where they fill pore spaces and reduce permeability. Engineers may perform formation tests to predict the likely effectiveness of treatments to remove these fines. The effects of completion choices may also be assessed using formation tests to aid engineers in planning required remedial operations.
Well and formation test data provide operators with information about their new and producing wells that is critical to making near-term operational decisions. But the real power of well test data is their application to construction or correction of reservoir models, which allow operators to make better long-term decisions about their assets.
Oilfield Review 2016.
Copyright © 2016 Schlumberger.
FAQs
What is meant by well testing? ›
PetroWiki. A “well test” is simply a period of time during which the production of the well is measured, either at the well head with portable well test equipment, or in a production facility.
What is the aim of well testing? ›The overall objective is identifying the reservoir's capacity to produce hydrocarbons, such as oil, natural gas and condensate. Data gathered during the test period includes volumetric flow rate and pressure observed in the selected well.
What are the methods used for well test analysis? ›The chapter explains six interpretation methods for well tests: (1) wellbore storage and skin effect, (2) outer boundaries and discontinuities, (3) well-test interpretation, (4) pressure-derivative analysis, (5) interpretation using simultaneously measured pressure and sand-face flow rate, and (6) wellbore storage ...
What measurement is from well testing? ›Production well testing refers to the process of performing periodic measurements of well productivity. Main parameters to be measured are flow rates (oil, gas, and water), pressures and temperatures (downhole and surface), fluid properties (density, shrinkage, SG, composition, etc.).
Why is well water testing important? ›Your well water should be free of microorganisms, such as bacteria, viruses or parasites that may cause disease, and from chemicals at levels that may be a risk to your health. If you have a private well, you should have the water tested to see if there are any problems.
What are the four types of well? ›There are water wells, oil wells, gas wells, and more. Wells have been used in many cultures around the world for over 8,000 years. The first wells were likely dug by hand or with very simple tools.
What is the best way to test well water? ›The best way to test your water is to send samples to a laboratory for testing, as this method provides the most accurate results. You can buy home testing kits, but you should primarily use these as a way to test water in between your laboratory checkups, as accuracy is important when it comes to safe drinking water.
What are the three testing methods? ›Basically, there are 3 testing techniques that are used for testing. They are White Box Testing, Black Box Testing, and Grey Box Testing. Each of the testing techniques is briefed below for your better understanding.
What are the four basic testing methods? ›There are four main stages of testing that need to be completed before a program can be cleared for use: unit testing, integration testing, system testing, and acceptance testing.
How long does well testing take? ›A typical inspection normally takes 1 ½ to 2 hours, and we run the water for up to an hour as needed.
What is critical flow in well testing? ›
Critical flow typically occurs when the pressure upstream of the wellhead is at least 70% higher than the pressure downstream of the wellhead, or when the ratio of downstream pressure to upstream pressure is 0.588 or less.
How do you measure well capacity? ›Turn on the well pump. Measure the flow rate as the time it takes to fill up the 5-gallon bucket. Discharge water at a rate of five gallons per minute for two hours (for a total of 600 gallons) or discharge water at a rate of four gallons per minute for four hours (for a total of 900 gallons).
How often should a well be tested? ›When to have your well tested. At a minimum, check your well every spring to make sure there are no mechanical problems; test it once each year for total coliform bacteria, nitrates, total dissolved solids, and pH levels. If you suspect other contaminants, you should test for those as well.
How do you know if your well is deep enough? ›If you cannot see the top of water in your well then you can tie a fishing float or "bobber" to your string and lower it carefully into the well until it stops dropping. Mark the string at ground level. Measure that string length - that's the depth from the ground surface to the top of your well water.
What causes a well to fail? ›A tank or pump failure can be caused by a number of factors: age, low-quality components, running without water, constant cycling or a clogged intake valve. Water pumps and pressure tanks don't need much in the way of maintenance, but they do need the right environment to reach their life expectancy.
What are the three tests for water? ›Physical parameters - testing for total and suspended solids. Metal parameters - testing for metal determines asset wear, system integrity and water quality (learn more about testing for metals here) Corrosion, scale and contaminants - reveals water quality, underlying system parameters and safety.
What is the most important water test? ›Bacteria Tests
One of the most common and most looked for is E. coli bacteria, which comes from fecal matter exposure and can result in serious health issues when consumed. Bacteria testing is essential in determining how safe water is to drink or expose to your skin.
In general, water testing can be classified as bacterio¬logical, mineral/inorganic and organic chemicals tests. Bacteriological tests generally check for indicator bacteria (for example, total coliform, fecal coliform or Escherichia coli) and can indicate the presence or absence of disease-causing bacteria.
What makes a good well? ›Dug wells should have a sealed casing and cover, and be located at least 25 feet away from ponds or streams. They should be uphill from and at least 100 feet away from sources of contamination including septic systems, livestock, and fuel tanks .
What are two kinds of wells? ›Types of wells
Dug wells have a large diameter, are shallow (approximately 10 to 30 feet deep) and are not cased continuously. Driven wells are constructed by driving pipe into the ground. Driven wells are cased continuously and shallow (approximately 30 to 50 feet deep).
What are the two types of well? ›
Generally, wells are of two types namely open wells and tube wells. Out of these two, tube well irrigation has become wide spread in India.
What is a safe level of coliform in well water? ›The Maximum Contaminant Level (MCL) for bacteria in drinking water is zero total coliform colonies per 100 milliliters of water as established by the EPA. The total coliform test is the basic yardstick for determining the biological quality in a water supply.
Why would a well fail water test? ›Well water contamination sources and coliform bacteria
Sometimes, however, these bacteria still make their way into your well. Usually this would be as a result of a cracked or improperly sealed well, or from contamination during service on the well, but sometimes the source of contamination remains a mystery.
- Check the Air Fill Valve. ...
- Have Your Pipes Inspected. ...
- Inspect the Water Itself. ...
- Inspect the Pump and Pressure Tank. ...
- Turn off the Water Supply. ...
- Drain the Tank. ...
- Remove and Replace the Tank.
- Testing shows the presence of defects, not their absence. ...
- Exhaustive testing is impossible. ...
- Early testing saves time and money. ...
- Defects cluster together. ...
- Beware of the pesticide paradox. ...
- Testing is context dependent. ...
- Absence-of-errors is a fallacy.
Survey the entire test prior to taking the exam. Take a few deep breaths and relax tense muscle - repeat throughout the test. Read directions carefully - ask questions. Answer easier questions first - this will help calm you down.
What are the two main types of testing? ›- Unit tests. Unit tests are very low level and close to the source of an application. ...
- Integration tests. ...
- Functional tests. ...
- End-to-end tests. ...
- Acceptance testing. ...
- Performance testing. ...
- Smoke testing.
- #1) Positive Attitude.
- #2) Good Communication.
- #3) Multi-Tasking Abilities.
- #4) Quick Learner.
- #5) Passion For Testing.
- #6) Team Player.
- #7) Think And Act As An End-user.
- #8) Analytical Abilities.
The Water Well Board suggests that a minimum water supply capacity for domestic internal household use should be at least 600 gallons of water within a two-hour period once each day. This is equivalent to a flow rate of 5 gallons per minute (gpm) for two hours.
How long does it take to drill a 100 foot well? ›Depending on the conditions of the weather, ground and water depth as well as drilling conditions, it typically takes 1 to 3 days, sometimes longer, to drill a well.
How many types of well testing are there? ›
Five test types are briefly discussed below: Pressure Build-Up, Injection/Fall-Off, Multi-rate, Multiple well, and Closed Chamber.
What is critical and normal depth? ›Normal depth is the depth of flow that would occur if the flow was uniform and steady, and is usually predicted using the Manning's Equation. Critical depth is defined as the depth of flow where energy is at a minimum for a particular discharge. Flow profiles are classified by the slope of the channel (So), yn, and yc.
What is critical depth in pipe? ›Critical depth is the depth at which the specific energy of a given flow rate is at a minimum. For a given discharge and cross-section geometry there is only one critical depth. The crown is the inside top of the culvert. The flowline is the bottom invert of a conduit.
What is critical pumping rate? ›In the context of corrosion or errosion, critical flow rate is the maximum flow rate that avoids damage to the pipe from corrosion or erosion. In the context of liquid unloading, critical flow rate is the minimum flow rate to produce liquids from a well.
What is a good well volume? ›Typical numbers for well recovery rates (if measured honestly over a 24-hour period) run from a fraction of a gallon per minute (a terribly poor well recovery or flow rate) to 3 gallons a minute of water flow (not great but usable) to 5 gallons per minute (just fine for residential use) to more than 10 gpm (a great ...
Is 7 gpm good for a well? ›The average American household needs 100 to 120 gallons per person per day, and a flow rate of about 6 to 12 gallons per minute. This requirement may be higher if it serves a home housing a large family or there are large water demands.
How do you calculate well function? ›...
- steady state flow conditions are not required.
- only one observation well is necessary.
- S can be determined.
- short pumping periods will generally suffice.
What is the Average Life Span of a Well? The average life span of an oil or natural gas well is 20 to 30 years. However, new technologies are being developed to find new ways to extend the life span. The life span of a well is based on the active years the well is in production.
How often does a well need to be cleaned? ›As a routine maintenance practice, clean your well at least once a year. If you have an iron or sulfur bacteria problem, clean more often.
Is it common to have coliform in well water? ›Various types of Coliform live in the soil and even on surfaces in your home, but they do not occur naturally in groundwater. If Coliform bacteria (sometimes reported as Total Coliform) are found in your well water, it is an indication that disease-causing bacteria could get in the same way.
What is the full meaning of well? ›
ˈwel. : an issue of water from the earth : a pool fed by a spring. : source, origin. : a pit or hole sunk into the earth to reach a supply of water. : a shaft or hole sunk to obtain oil, brine, or gas.
What are the three types of well? ›There are three types of wells: dug, driven, and drilled.
What is well example? ›We use well as an adverb when something is done to a good standard or in a good way: He drives very well. I work very well late at night.
What are two meanings of well? ›When well is a noun, it means "a deep hole full of water or oil." When well is an adverb, it describes the way something's done. If you're not sure when to use well and when to use good, think about what you're describing.
What is well well well means? ›Interjection. well, well, well. Indicating pondering or consideration, often with sarcasm or mock surprise. Well, well, well. What have we here?
What are the stages of a well? ›This includes all stages of a well's life cycle: exploration, development and operation, abandonment and reclamation.
What are the causes of well failures? ›Causes of Well Problems
Improper well design and construction ● Incomplete well development ● Borehole stability problems ● Incrustation build-up ● Biofouling ● Corrosion ● Aquifer problems ● Over-pumping.
- hardness.
- high iron content.
- bad odor.
- taste.
- color.
- high solids content.
- presence of bacteria, lead, cysts, radon, arsenic, nitrates, metals and other health-affecting materials.
Broadly, water wells can be classified into four groups according to their functions: (a) water supply wells, (b) recharge wells, (c) drainage wells, and (d) monitoring wells.