Evaluation of the M-Site B-Sand Fracture Experiments: The Evolution of a Pressure Analysis Methodology
Authors: Gulrajani Sunil N. , Nolte Kenneth G. , Romero Jean 
Published: December 2007

The most comprehensive hydraulic fracturing data including the first objective measurements of the fracture height, length and width are those acquired from the GRI/DOE M-Site tests. In spite of the availability of extensive and reliable fracturing data, significant deviation between the predicted and microseismic-determined fracture geometry was reported. The purpose of this study is to provide a consistent analysis of the B-sand experiments by applying a systematic methodology for fracture treatment evaluation.

For this analysis, the fracture parameters are initially estimated from laboratory data, well logs and calibration tests. These parameters are subsequently refined by matching simulated pressures to field-measured fracturing pressures recorded during the first linear gel injection. These fracture parameters are then used to compare predicted and measured fracture pressures on all subsequent injections. Although general agreement for the fracturing pressures was obtained, a discrepancy was noticed between the zone stresses estimated by the evaluation and their variation as indicated on published stress logs. The stress data were reinterpreted and an acceptable pressure match was established.

The fracture parameters resulting from this study are in agreement with independently inferred estimates. In addition, an apparent difference between closure pressure and microfrac stress is resolved. Finally, good agreement between the predicted fracture geometry and the microseismic readings is observed for each injection test considered in this study. This study thus shows that fracture pressures and geometry can be consistently predicted with good accuracy using elementary analysis techniques, without a reliance on ad hoc physical explanations.

Background

Over the past decade, a series of hydraulic fracturing experiments, jointly conducted by the Gas Research Institute (GRI) and the Department of Energy (DOE) at the Multi-Well Site (M-Site), has provided the most comprehensive data available for hydraulic fracture treatments. The initial objective of these experiments was to establish the character of gas production from lenticular, low-permeability formations which are common in the Western United States. Through the course of the experiments, the focus has evolved toward developing methodologies to increase the accuracy for measurement of field-scale hydraulic fractures. The primary effort in this direction has been the successful use of subsurface triaxial accelerometers to locate microseismic events along the extent of a propagating hydraulic fracture. This objective measure of fracture dimensions and other supporting fracturing data provide the critical constraint for evaluating fracture models and thus provide an excellent example for comprehensive fracture evaluation.

In spite of the availability of such exhaustive and reliable fracturing data, widely used fracture simulators failed to comprehensively explain the observed fracture response for this important data set. This discrepancy for the B-Sand experiments was reported when using both cell-based and lumped 4 fracture simulators. Although net pressures were matched for the calibration treatments, disagreement was noticed between the simulated fracture geometry and the geometry outlined by the microseismic measurements. The disparity in fracture geometry was particularly pronounced on the propped treatment for which not even a satisfactory net pressure match was achieved.

An undesirable feature of this lenticular formation is the complex geological environment that is prone to inefficient hydraulic fracturing. A comprehensive list of factors responsible for abnormal fracture behavior was discussed by Nolte. A majority of these characteristics are applicable to the in-situ conditions at the M-Site, leading to its classification as the "worst-case scenario." P. 115^