Determination of the Compass Orientation and Length of Hydraulic Fractures by Pulse Testing

Abstract

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

Pulse Testing With Wellbore Storage and Skin Effects

“…Verify the design data by ensuring that the values t LOI At ct =0.18 and t LI At ct =0.31 are within the range of the KB correlations, and by ensuring that the expected amplitude of about 3.6 psi [25 kPa] can be measured with the available pressure gauges. We conclude that the well can be pulse tested at the rate of 50 BID [7.95 m 3 Id] for about 5 hours, shut in for another 5 hours, and pulsed again. The expected pressureresponse amplitude from such a test will be about 3.6 psi [24.9 kPa] and the time lag will be about 3 hours.…”

“…Pulse tests also provide a qualitative indication of reservoir heterogeneity, fracture compass orientation, and fracture lengths. 2,3 One advantage of pulse testing over conventional interference testing is the graphic technique of pulse-test analysis. With this technique, the pressure response caused by the flow disturbances can be distinguished from other reservoir pressure trends and noise.…”

Determining Fracture Orientation From Pulse Testing

“…Two previous studies 7,8 presented techniques that could be used to determine fracture orientation and/or fracture length of a vertically fractured well from pulse testing. These methods have certain limitations and this study has overcome some of them.…”

“…7 described a method to use pulse tests to determine both the compass orientation and the length of hydraulic fractures. The method, however, requires both post-and prefracture pulsetest parameters and thus cannot be applied to wells that already have intersecting fractures.…”

Modifying subterranean fluid flow patterns using barriers to improve pre-drainage in coal mining

“…In addition, during a treatment the pressure observation wells can be used to potentially observe the approach and intersection of a hydraulic fracture by a observing the characteristic pressure decrease predicted as the propagating fracture tip approaches the well then sudden increase to the minimum horizontal stress as the hydraulic fracture passes beyond the observation well (Branagan et al, 1998;Branagan et al, 1997;Jaeger and Cook, 1969). Over time, pressure observation wells can be used to characterise the direction and drainage area of the fracture by transient testing using interference principles between one or more observation wells (Mavor 26 and Robinson, 1993;Meehan et al, 1989;Pierce et al, 1975;Sawyer et al, 1980;Uraiet et al, 1977).…”