Procedures for the analysis of drill stem test pressures are developed. They are based on the solution for radial flow of a slightly compressible fluid from a radially infinite, homogeneous and isotropic reservoir into a finite well with a "skins". They differ from previously used methods in that they take account of variations in flow rate throughout the flowing period.
The formulas cover the cases of measured and unmeasured flow rates; a short flow period with a relatively long shut-in, comparable flow and shut-in periods, and multiple shut-in tests. They solve for fully built-up pressure, formation flow resistance (transmissibility) and skin pressure, formation flow resistance (transmissibility) and skin resistance, and for flow rates when these are not measured.
The procedures are quick and easy to apply, given the appropriate pressure measurements, although checking the validity of results lengthens pressure measurements, although checking the validity of results lengthens the calculations appreciably in one case. It appears that highly precise results are obtainable by these methods for most drill stem tests causing smoothly varying single phase flow in the reservoir.
The purpose of this paper is to present methods for the analysis of drill stem test pressures, allowing for the large changes in flow rate which normally occur but are seldom measured. The analysis derives the parameters required both to predict performance for the interval tested parameters required both to predict performance for the interval tested and to estimate the pressure gradients caused by flow in the reservoir nearby.
Detailed analyses of drill stem tests have been made at the BP Research Centre and have shown that:
the peak flow rate usually is two to ten times the final flow rate.
this flow rate variation has a considerable effect on the subsequent pressure build-up.
the build-up is very sensitive to the final flow rate and the total production but is poorly sensitive to early flow rates.
the rate of rise of pressure at the and of a flow period usually differs significantly from the rate of increase of the static head in the drill pipe, even for very low gas-oil ratio fluids such as water, when flow rates exceed a few tens of barrels per day.