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 tester through a probe inserted into the formation or between packers set above and below the sampling site.