Field Application of Unsteady-State Pressure Analyses In Reservoir Diagnosis
Authors: Morse J.V. , Ott Frank 
Published: December 2007

Unsteady-state pressure analyses of buildup and fall-off tests provide useful quantitative values for reservoir properties, and the simplicity of obtaining and analyzing the tests makes them valuable diagnostic tools. This technique has been routinely applied in waterflood injection wells and in both pumping and flowing producers in primary and secondary recovery operations. These analyses indicate reservoir permeability, skin damage, permeability barriers, reservoir pressure and, in many instances, location of flood fronts. Typical field examples are presented.

Introduction

Unsteady-state pressure analyses of buildup and fall-off tests have long been recognized as a valuable and reliable tool for determining reservoir flow characteristics. This one topic has received much essential research since it was first widely publicized some 15 years ago. This article does not present new theory but emphasizes those techniques and formulas felt to be most applicable to routine reservoir analysis. Several typical field examples are presented to exemplify the relative simplicity of the technique and to encourage a more widespread use of the method as a routine diagnostic tool. During recent years the Monahans production district of Humble Oil and Refining Co. has analyzed some 400 pressure buildup and fall-off tests in practically all phases of greater than and gas-producing operations. In secondary recovery waterfloods, routine pressure fall-off analyses in water injection wells are used to determine if injection is restricted by wellbore plugging or formation damage. These tests are instrumental in the design of well stimulation treatments and, equally important, needless workovers are eliminated in cases where no formation damage is indicated, demonstrating that the wellbore is in good communication with the formation. It has been found that fall-off tests in injection wells will often give a reasonable estimate of the flood front location. These same tests have also been used to determine that undesirable zones or channels are thieving injection water, and have been valuable in analyzing problem or water breakthrough areas in floods. Surface pressure recording equipment normally can be used, facilitating the testing procedure. Satisfactory analyses have been made in pumping wells in which buildup tests are difficult to obtain by injecting fluid into the well and obtaining fall-off tests. As in injection well tests, fall-off tests in pumping wells can verify the existence and approximate radial extent of damage, indicate effectiveness of stimulation treatments and show reservoir permeability and pressure. Pressure buildup tests are normally obtained in flowing oil wells using subsurface pressure recorders. These tests, when properly planned, provide essential data related to damage, reservoir permeability and pressure, and existence of permeability barriers within the drainage area. Application of this latter information can prove valuable in development drilling programs by indicating high-risk locations. Careful analyses of pressure buildup and fall-off tests can permit better evaluation and surveillance in secondary recovery operations, are beneficial in optimizing well stimulation design and permit better evaluation of possible offset drilling locations.

Theory

Unsteady-state pressure behavior is observed in a well in an infinite reservoir or in a bounded reservoir at times before the effects of the reservoir limits are felt. The basic relations used in this article are solutions to the continuity equation for a model assuming a homogeneous, horizontal reservoir with infinite radial extent and completely filled with a fluid of constant compressibility. The solution also assumes that the well has been produced at a constant rate since time zero.

JPT

P. 869ˆ