BUILD UP PRESSURE TEST ANALYSIS

The use of pressure buildup data has provided the reservoir engineer with one more useful tool in the determination of reservoir behavior. Pressure buildup analysis describes the buildup in wellbore pressure with time after a well has been shut in. One of the principal objectives of this analysis is to determine the static reservoir pressure without waiting weeks or months for the pressure in the entire reservoir to stabilize. Because the buildup in wellbore pressure will generally follow some definite trend, it has been possible to extend the pressure buildup analysis to determine:

• the effective reservoir permeability;
• The extent of permebility damage around the wellbore.
•  The presence of faults and to some degree the distance to the faults;
• Any interference between producing wells.
• The limits of the reservoir where there is not a strong water drive or where the aquifer in no longer than the hydrocarbon resrvoir.

Certainly all of this information will probably not be available from any given analysis, and the degree of usefulness of any of this information will depend on the experience in the area and the amount of other information available for correlation purposes.

The general formulas used in analyzing pressure buildup data come from a solution of the diffusivity equation. In pressure buildup and drawdown analyses, the following assumptions, as regards the reservoir, fluid, and flow behavior, are usually made:

• Reservoir: homogeneous; isotropic; horizontal of uniform thickness.
• Fluid: single phase, slightly compressible, constant U0 and Bo.
• Flow: laminar flow, no gravity effects.

Pressure buildup testing requires shutting in a producing well and recording the resulting increase in the wellbore pressure as a function of shut-in time. The most common and simplest analysis techniques require that the well produce at a constant rate for a flowing time of Tp, either from startup or long enough to establish a stabilized pressure distribution, before shut in. The pressure is measured immediately before shutin and is recorded as a function of time during the shut-in period. The resulting pressure buildup curve is then analyzed to determine reservoir properties and the wellbore condition.

Stabilizing the well at a constant rate before testing is an important part of a pressure buildup test. If stabilization is overlooked or is impossible, standard data analysis techniques may provide erroneous information about the formation. Two widely used methods are discussed below; these are:

• The Horner Plot.
• The Miller_Dyes_Hutchinson method.

HORNER PLOT

A pressure buildup test is described mathematically by using the principle of superposition. Before the shut-in, the well is allowed to flow at a constant flow rate of Qo STB/day for Tp days. At the end of the flowing period, the well is shut in

with a corresponding change in the flow rate from the “old” rate of Qo to the «new» flow rate of Qnew =0.

Calculation of the total pressure change which occurs at the sand face during the shut-in time is basically the sum of the pressure changes that are caused by:

• Flowing the well at a stabilized flow rate of Qold, for example, flow rate before shut-in Qo, and is in effect over the entire time of tp+t,
• The net change in the flow rate from Qo to 0 and is in effect over Δt.