DRAWDOWN TEST OBJECTIVES:The objectives of a drawdown test are to determine skin, perm and the distances to the reservoir’s boundaries. We recommend doing a drawdown test to look for reservoir limits instead of running a build-up because a flowing well test does not interrupt cash flow. Since you can’t see a boundary until the pressure wave hits it, there’s no way to tell how long a build-up is required to see the reservoir limits. On most tests, it is possible to perform an «Integrated Volume Explored» calculation as the test progresses. This allows for a test to be run until a given amount of reserves have been proven.
Important reservoir parameters can be determined by flowing a well at a constant rate and measuring flowing wellbore pressure as a function of time. This is called drawdown testing and it can utilize information obtained in both the transient and pseudo-steady-state flow regimes.
If the flow extends to the pseudo-steady state, the test is referred to as a reservoir limit test and can be used to estimate in-place gas and shape of the reservoir. Both single-rate and two-rate tests are utilized depending on the information required. The purpose of the drawdown testing is to determine the reservoir characteristics that will affect flow performance. Some of the important characteristics are the flow capacity kh, skin factor s, and turbulence coefficient D.
CHARACTERISITICS OF FLOW AND GAS WELL TRANSIENT TESTING.
Much of that information can be obtained from pressure transient tests. Pressure transient testing techniques, such as builup, drawdown, interference, and pulse, are important part of reservoir and production engineering. As the term is used in this book, pressure transient testing includes generating and measuring pressure variations with time in gas wells and subsequently, estimating rock, fluid, and well properties and predicting reservoir/ well behaviour. Practical information obtainable from transient testing includes wellbore volume, damage, and improvement; reservoir pressure; permeability; porosity; reserves; reservoir and fluid discontinuities; and other related data. All this information can be used to help analyze, improve, and forecast reservoir performance.
Pressure interference or pulse testing could establish the possible existence and orientation of vertical fracture of a gas reservoir. However, other information (such as profile surveys, production logs, stimulation history, well production tests, packer tests, core descriptions, and other geological data about reservoir lithology and continuity) would be useful in distinguishing between directional permeability and fractures or estimating whether the fractures were induced or natural.
CHARACTERISITCS OF VARIOUS FLOW REGIMES:
- TIME-HISTORIES FOR A CONSTANT-RATE DRAWDOWN TEST
EARLY-TIME FLOW REGIME:
Initially during early-time flow, wellbore storage and skin effects dominate the flow. When the well is opened at the surface for flow at a constant rate, the initial flow comes primarily from the wellbore itself, rather than from the formation. In fact, flow from the reservoir increases gradually from zero until the specified wellhead flow rate Q is reached in a length of time.
TRANSIENT FLOW REGIME:
In this flow regime the pressure is the same as that created by a line-source well with a constant skin. Since a plot of PD versus To (Time) on semilogarithmic coordinates will yield a straight line, the analysis of transient flow data is often referred to as a semilog analysis. The semilog analysis of drawdown data yields consistent values of reservoir parameters. Only the permeability thickness kh, the skin factor s, and the inertial-turbulence factor D may be determined from such an analysis. This semilog straight line continues as long as the reservoir is infinite-acting. If a fault is encountered in the reservoir, the slope of the line will double, and a new straight line will be established.
When the reservoir boundary begins to have a significant effect on well drawdown, the transient region ends; the pseudo-steady-state or depletion phase directly follows the transient period.
When a constant-rate drawdown test is run for a long period of time, the boundary effects eventually dominate the pressure behavior at the well. The pressure starts declining at the same rate at all points in the reservoir; hence the name pseudo-steady-state. In effect, then, the total drainage area is being depleted at a constant rate. A plot of ApD versus to on arithmetic cordinates will yield a straight line from which the reservoir pore volume occupied by gas and the reservoir limits can be calculated. Tests utilizing this regime of the drawdown history are often known as reservoir limit tests.
USES OF PRESSURE DRAWDOWN TESTS:
Producing the well at a constant flow rate while continuously recording bottom-hole pressure runs the drawdown test. In this type of test, wellcompletion data details must be known so the effect and duration of wellbore storage may be estimated. While most reservoir information obtained from a drawdown test also can be obtained from a pressure buildup test, there is an economic advantage to drawdown testing since the well is produced during the test. Properly run drawdown tests may provide information about formation permeability K, skin factor, S, and the reservoir volume communicating with the well. The main technical advantage of drawdown testing is the possibility for estimating reservoir volume. The major disadvantage is the difficulty of maintaining a constant production rate.
- DRAWDOWN TESTS