Realistic Proppant Conductivity Data Improves Hydraulic Fracturing Treatment Design
Authors: McDaniel B.W. , Hoch O.F. 
Published: March 2010

Successful application of hydraulic fracturing is dependent on obtaining the desired propped fracture length and conductivity. Post-frac pressure analysis methods are often used to calculate the apparent length and conductivity that were achieved by a fracturing treatment. Many times, the calculated conductivity values are a factor of two to five lower than had been predicted by the pre-frac design calculation. Occasionally values an order of magnitude lower than those predicted are found.

Recent laboratory tests on proppants held at stress for long time periods at various reservoir temperatures resulted in much lower conductivity values than when generated with very short test times. The result of this testing program has been the development of a large base of conductivity data that are much more realistic than the data which have been in use for many years. This may help explain discrepancies in pre-frac predictions and post-frac calculations. In many cases, the treatment design will be altered appreciably by the use of more realistic conductivity data.

In this paper, we will present comparisons of the old data and the more realistic values. We will also illustrate the significant effect that use of the new data can have on fracturing treatment design. In many applications the recommended proppant addition schedule will be much more ambitious. Some well conditions may result in the need for a stronger propping agent than had been predicted using less realistic conductivity data.

Introduction

In recent years, advances in the field of transient pressure analysis on hydraulically fractured wells will often allow an operator to make post-frac estimates of effective fracture length and fracture conductivity(1,2). No longer are the analyses based on "infinite conductivity" fractures where the calculated fracture length was typically less than 30 m. When these new "finite conductivity" analysis methods were first introduced, the fracture length and fracture conductivity values were often in significant disagreement with pre-frac predictions of computer design simulator models. More recently, refinements in analysis techniques, coupled with further experience in applying these methods, have resulted in improved calculations which yield a calculated fracture length that is usually in reasonable agreement with design simulators(3.4). This has not been the case with post-frac calculations of fracture conductivity, however, where pre-frac predictions continue to be much higher in many cases(3–6).

In 1973, Cooke(7) published data indicating that brine and temperature will have a significant effect on fracture conductivity with time. Corrosion of test equipment and other problems interfered with later attempts to extend this work. The net result was that essentially all the conductivity data published by the industry has been at ambient test temperatures with short test times at each stress level(8,11). A few authors have tested at higher temperatures, but again using only short time durations at each closure stress(12,13).

In 1986, McDaniel(14) published work that described how these experimental problems can be overcome. The paper also presented data that indicated "time at stress" may be a very important factor at any temperature, although the primary emphasis was at relatively high reservoir temperature (135 °C).