The Stimulation Treatment Pressure Record An Overlooked Formation Evaluation Tool

Abstract

Pressure transient analyses of stimulation treatment pressure records Pressure transient analyses of stimulation treatment pressure records give formation permeability and show the condition of the formation around the wellbore both before and after treatment. Introduction Much has been written in recent years about transient testing theory and its application in understanding well behavior. The literature illustrates various special transient analysis techniques and their interpretations; however, in the past, one important source of transient data has been overlooked. This source is the stimulation treatment pressure record. On every well that is stimulated by a service company, a treatment report giving pertinent well data, materials and volumes, and pumping times is prepared, and a copy is presented to the oil company representative. In nearly every case, the treating pressures are recorded by either a strip chart or a circular clock recorder. Either the original pressure chart or a copy is given to the customer if he requests it. During the treatment, fluids are injected into the well, causing a pressure response that is recorded at the wellhead. At times the injection may be stopped for mixing of fluids. These injection pressure buildups and fall-offs during the treatment can be calculated and interpreted to determine the condition of the wellbore and the formation transmissibility. The engineer can check the validity of the result by calculating expected production before and after the treatment and comparing with the actual well production. Statement of Theory and Definitions In the analysis of drawdown pressures at a constant well production rate in an infinite reservoir, the point source solution is used. This equation is also valid for describing the behavior of a well in a bounded reservoir prior to the occurrence of any boundary effects. Except for the sign of the production rate, q, the same transient equation applies to well pressure buildup at a constant injection rate. Although this is well known mathematically, it has not been used in actual testing in our experience because of the supposed problem of holding a constant injection rate and because of possible filtering at the formation face in the wellbore. possible filtering at the formation face in the wellbore. In actual practice, though, as we will see later, these problems are not a real handicap in most wells particularly problems are not a real handicap in most wells particularly during early injection times. All the same equations developed for constant rate drawdown can be applied to constant injection rate buildup. An advantage of using a constant injection rate buildup test is that the liquid injected is not as compressible as most produced fluids; therefore, the wellbore unloading factor found in producing wells is not significant in an injection test. producing wells is not significant in an injection test. The following approximate equations (Eqs. 2 and 3) are well known and apply when tdw = (0.000264kt/ccrw2) greater than 100. JPT P. 951