Determination of the Compass Orientation and Length of Hydraulic Fractures by Pulse Testing
Authors: Pierce A.E. , Vela Saul , Koonce K.T. 
Published: December 2007

Field tests have shown that compass orientation and the length of hydraulic fractures can be determined by pulse testing in different directions from wells before and after fracturing. The method determines orientation by sampling a large portion of the reservoir, is applicable to cased holes, and provides an estimate of fracture length.

Introduction

It is often desirable to know the flow patterns created after a number of wells in a reservoir have been fractured. This requires a knowledge of the compass orientations, lengths, and conductivities of hydraulic fractures. Standard pressure interference testing has been used to determine the orientation of natural fractures and inflatable impression packers and television cameras have been used to locate hydraulic fractures in open-hole completions. None of these methods, however, determines both the compass orientation and the length of a hydraulic fracture, and only one method samples a large portion of the reservoir. This paper describes a way to use pulse testing to determine both the compass orientation and the length of such fractures. Results from two field tests are presented. presented. Theory

Johnson et al. give a complete description of the procedure and technology of pulse testing. The method procedure and technology of pulse testing. The method involves changing the rate of flow at one well and measuring the pressure response at one or more offset wells. The response, or pressure pulse, is characterized by two parameters, the time lag and the pulse amplitude. These and other pulse-testing parameters are illustrated in Fig. 1.

Pulse amplitude depends on flow rate, pulse interval, between-pulse interval, and, to some extent, on reservoir properties, transmissibility, and storage. Time lag properties, transmissibility, and storage. Time lag primarily depends on transmissibility and storage. It has primarily depends on transmissibility and storage. It has been shown that the presence of a high-transmissibility zone or of a zone of very low transmissibility (a barrier) can be detected by pulse testing. A fracture, of course, creams a zone of high transmissibility with an insignificant change in storage as compared with that of the unfractured matrix. Fig. 2 shows the relationship between transmissibility (for a fixed value of storage) and both time lag and response amplitude. Clearly, a change in time lag is sensitive to a change in transmissibility. Pulse amplitude, on the other hand, varies directly with Pulse amplitude, on the other hand, varies directly with changes in transmissibility over part of the range; but for greater than 3 x 10 md-ft/cp, the amplitude responds weakly to increases in transmissibility. For these reasons, changes in time lag should be most effective for determining the direction and length of a hydraulically induced fracture.

To evaluate the feasibility of this procedure, a model of a reservoir (see Appendix) was constructed with one pulsing well in the center and several responding wells pulsing well in the center and several responding wells around the pulser. A pulse test was then simulated by producing and shutting in the pulsing well intermittently producing and shutting in the pulsing well intermittently for equal increments of time. The pressure response was computed for all responders and was then analyzed using the tangent method to determine the time lags. A hydraulic fracture was then simulated by narrow blocks with high but finite permeability extending an assumed length from the well. The same pulsing sequence was repeated and the results were analyzed for time lags after fracturing. The ratio of the time lag before fracturing to the time lag after fracturing was plotted as a function of direction (angle) around the pulser.

JPT

P. 1433