1. Introduction

Analytical models and numerical simulation are important means to increase understanding and to enhance efficiency of hydraulic fracturing. The reasons for developing and using them are clearly explained, for instance, by Mack and Warpinski [1]. Thus, there is no need to dwell on them. Rather we focus on improving analytical and numeric methods used to the date. Our objective is to suggest new approaches for developing accurate, robust and stable simulators on the basis of recent analytical and computational findings [2–6].

The approaches discussed in the paper stem from the fact [2,3] that the conventional formulation of the hydraulic fracture problem (for example, see [7]), when neglecting the lag and fixing the position of the fracture front at a time step, is ill-posed. This feature has not been reported for more than three decades of studying hydraulic fractures because of two rea-sons. Numerical simulators, based on the conventional formulation (for example, see [7,8]),employ quite rough meshes, which themselves serve as specific ‘regularizators’. On the other hand, rare solutions of model problems either also employed rough meshes [9], or they were obtained by solving the initial value (Cauchy) problem [10–12] rather than the boundary value problem (a discussion of the difference may be found in references [3,4]). The disclosure of the mentioned fact has led to (i) explicit formulation of the speed equation (SE) in its general form1, (ii) comprehension of its significance for proper numerical simulation of hydraulic fractures and (iii) distinguishing the particle velocity as a preferable variable2. It has also led to the efficient means, called ε–regularization [2,3], to overcome the analytical and computational difficulties caused by ill-posedness. Finally, the entire conventional formulation of the problem has changed to the modified formulation, which opens new analytical and computational options for solving hydraulic fracture problems.

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