Pulse-Testing Of Vertically Fractured Wells

Abstract

Abstract Generalized correlations are presented with which an engineer may analyze and design pulse tests in fractured wells. The results of this study can be used to obtain the compass orientation of a vertical fracture as well as the degree of reservoir anisotropy by pulse tests provided the formation permeability and fracture length are known. These two parameters may be obtained from a pressure test at the fractured well. On the other hand, if only the permeability is known, then the results may be used to estimate fracture length and determine orientation. The main advantage of the approach suggested here is that prefracture data are not needed. prefracture data are not needed. Conditions necessary to insured that the response obtained is maximum are described. Thus, appropriate instruments, pulse and shut-in periods, etc., can be chosen to insure that the test is a success. INTRODUCTION AND OBJECTIVES The success of many depleted reservoirs and secondary recovery projects can be attributed directly to hydraulic fracturing. The use of this technique then required many studies to examine the pressure behavior at a fractured well in order to pressure behavior at a fractured well in order to determine formation flow capacity, fracture length, and compass orientation. Recently, due to the initiation of costly enhanced recovery programs, the compass orientation has become a point of serious concern. The principal objective of this paper is to describe a method of determining fracture orientation by pulse testing. Methods proposed in the literature to determine the compass orientation of vertical fractures include the use of inflatable impression packers and television cameras core analysis, geophysical techniques, and pressure transient analysis. The first three methods sample only a small volume of the reservoir and, thus, do not reflect conditions at large distances from the wellbore. Pressure transient tests offer the most potential since they sample a larger portion of the reservoir volume. Methods suggested in the literature to determine fracture orientation by pressure transient tests include: pulse testing, mapping the pressure distribution at pulse testing, mapping the pressure distribution at observation wells during injection of fluid at the fractured well, and interference testing. The method of Pierce et al. requires both prefracture and postfracture data. The advantage of prefracture and postfracture data. The advantage of the method suggested here is that prefracture data are not required, and thus it is directly applicable to wells that already intersect fractures. This situation is probably most representative of field conditions during or subsequent to waterflood operations. To use the generalized correlations presented here to estimate the compass orientation of the fracture and reservoir anisotropy, the formation permeability and fracture length must be known. permeability and fracture length must be known. On the other hand, if only the permeability is known, then the fracture length can be calculated and the compass orientation can be determined. The pressure response due to pulsing the fractured well is also presented. Therefore, this type of information has not been available and should be useful in choosing the necessary equipment to measure the expected response and in locating appropriate observation points. Furthermore, unlike Pierce et al., we have considered cases where the Pierce et al., we have considered cases where the pulsing period is not equal to the shut-in period. pulsing period is not equal to the shut-in period. The magnitude of the distances between the two wells considered here is also significantly different from that investigated in Ref. 8. The advantages of pulse testing over interference testing are also documented. THEORY The theoretical considerations and assumptions required to obtain the desired results may be conveniently discussed in two parts:the assumptions required to calculate the characteristic parameters of a pulse test, andthe assumptions parameters of a pulse test, andthe assumptions involved in selecting a model of the reservoir and fracture to obtain the required pressure data.