Hydraulic fracturing is a technology that has been utilized for more than 50 years in the oil and gas industry. It was originally used for stimulating hard, brittle formations which typically exhibited low permeabilities and roughly behaved as linear elastic materials. Nonetheless, an increasingly important segment of the industry is currently stimulating very soft and poorly consolidated formations; where the assumptions of ideal elasticity and relatively small fluid leak-off fail to hold (e.g. Gulf of Mexico, West Africa, Alaska, East China Sea). In these rock types, hydraulic fracturing stimulation has been mostly used to control and solve critical production Problems such as sanding and formation damage (caused during completion and/or drilling operations).
Most hydraulic fracturing projects carried out in unconsolidated formations render rather unexpected results: standard numerical models tend to underpredict fracturing pressures. A recent worldwide survey on fracturing pressures by the Del ft Fracturing Consortium (Papanastasiou, 1997) indicated that net pressures encountered In the field commonly are 50% to 100% higher than their corresponding values predicted by conventional fracturing simulators; the difference is even higher for the case of poorly-consolidated formations (Pak, 1997). he irnplementation of hydraulic fracturing operations in this type of rocks has not been accompanied by modeling techniques tailored specifically for this type of formations. In most cases, such models undergo a period of "calibration", in order to reproduce the results obtained in the field. Thus, a trial-and-error approach is commonly used to design and perform the treatments, avoiding major operational problems although without optimizingthe field operation.
This study aims at determining the importance of shear as a failure mechanism during hydraulic fracturing processes involving highly-permeable, poorly-consolidated rocks. This modeling work consisted of two main phases:
a discrete element approach to mimic both the mechanical and hydraulic behavior of an unconsolidated rock sampled from the field; and
a field model to infer the behavior of the rock modeled in the previous step during high pressure fluid injection.
In standard simulators, the traditional uni-dimensional fluid leak-off model is assumed to describe the fluid loss process from the fracture into the formation (Geerstma and de Klerk 1969). Nonetheless this assumption fails to hold for the case of highly permeable rocks, where radial flow is more likely to occur between the reservoir and the perforations. Likewise, assumptions regarding elasticity and infinitesimal deformation are not applicable due to the elasto-plastic nature of the rock. In this paper, PFC3D, a discrete element simulator was used to model the problem of hydraulic fracturing in highly permeable formations. The discrete element method (OEM) allowed for both tensile and shear failure to occur; it also had the ability to account for rock elastic and plastic deformations, as well as for poroclastic effects.
The models built in the context of this research work were aimed at describing both the mechanical and the hydraulic properties of typical poorly consolidated formations. Thus, high porosity, high permeability, low mechanical strength rocks were used for calibration purposes.