The current type-curve matching technique is essentially a trial-and-error procedure without any independent and accurate way of checking the validity of the end results.
This paper introduces a new technique for interpreting pressure tests. This technique uses log-log plots of the pressure and pressure derivative versus time to calculate reservoir and well parameters WITHOUT type-curve matching. This paper concentrates on the interpretation of pressure tests in which wellbore storage and skin are present. The technique essentially consists of obtaining characteristic points of intersection of various straight line portions of the pressure and pressure derivative curve, slopes and starting times of these straight lines. These points, slopes and times are then used with appropriate equations to solve directly for permeability, wellbore storage and skin. A step-by-step procedure for calculating these parameters without type-curve matching for five different cases is included in the paper.
The most important aspect of this new technique is undoubtedly its accuracy because it uses exact analytical solutions to calculate permeability, skin, and wellbore storage. The second most important feature of this new technique is that it is verifiable. Any two parameters calculated from two independent equations corresponding to two different portions of the pressure derivative curve are verified by a third equation which corresponds to a known and unique point relating the two parameters. The proposed technique is applicable to the interpretation of pressure buildup and drawdown tests. This technique is illustrated by several numerical examples.
Interpretation of pressure tests for a single well with wellbore storage and skin in a homogeneous reservoir considerably improved when the type-curve matching technique was published in the seventies. Later that decade Tiab introduced the pressure derivative analysis. He showed that a log-log plot of pressure derivative versus time is an important tool in identifying flow regimes and boundary effects. In the eighties type-curves which combine both the pressure and pressure derivative functions for various reservoir systems became an integral part of modern well test analysis. Unless all flow regimes are definitely observed in the pressure derivative curve, type-curve matching is still a risky technique. Also, combinations of various boundary conditions may yield approximately similar pressure behavior. For a well producing from a bounded system, it is possible for inner and outer boundary effects to interact and considerably affect the well pressure behavior such that the infinite acting radial flow line is either too short or non-existent. Horne showed that log-log type curve matching is not as accurate as conventional semilog methods, because log-log axes tend to mask inaccuracies at late time, where 1 mm deviation of a pressure point may mean an actual error of 200 psia. Finally, the noise in the pressure derivative curve can be sever enough to make it impossible to draw the characteristic straight lines corresponding to flow regimes.