Abstract

Observed reductions in the permeability of propped hydraulic fractures are examined by considering the role of mechanical stresses and the chemistry of pore fluids at elevated temperatures as agents of proppant diagenesis. Stress-enhanced dissolution of proppant increases the density of grain packing and reprecipitation of mineral matter further occludes pores – together these mechanisms additively reduce porosity and permeability. Experiments and analyses are presented which explore the evolution of porosity and permeability in proppant packs subjected to reservoir conditions. Experiments are completed in two modes: in flow-through reactors absent intergranular stresses to evaluate rates of dissolution and reprecipitation on proppant surfaces; and in uniaxially stressed reactors with stagnant fluids to evaluate the relative role of stress in mediating dissolution and porosity reduction. Lumped parameter models are used to evaluate rates of dissolution and chemical compaction in a range of proppants. Mechanisms include mineral dissolution, transport, and re-precipitation of the resulting products in the particle interstices, resulting in a loss of intergranular porosity. The model uses thermodynamic data recovered from the reactor experiments to constrain the projected loss of permeability for the mineralogical composition of available proppants.

1. INTRODUCTION

Hydraulic fracturing is a common method of stimulating wells to efficiently recover hydrocarbons from low permeability reservoirs. As the hydraulic fracture is driven by the injection of fluids, sized particles (i.e., proppants) mixed with fracturing fluids are injected to hold fractures open. This ultimate goal of this treatment is to create a high-permeability and high-surface-area conduit that may access the fluids within the reservoir. These stimulations are expensive, and this investment desires that the fracture treatment would remain a high-permeability conduit throughout the lifetime of the reservoir. However, recent evidence suggests that the performance of such treatments may be degraded early within the lifetime of production. These effects include proppant flowback, fines intrusion, proppant crushing, and proppant diagenesis [Weaver et al., 2005]. Moreover, the internal or innate effects of the proppants utilized such as types of proppants and proppant concentration, also control the fracture conductivity. These factors influence the overall effectiveness of the fracture treatment.

Fracture diagenesis, which is a physiochemical phenomenon, may exert a long-term influence on the fracture permeability among the above external factors. In this work we address mainly this phenomenon that may be a dominant mechanism influencing a long-term change in the fracture permeability (i.e., the workability and durability of wells). One of the main mechanisms of diagenetic compaction and deformation in sedimentary rocks is pressure solution [e.g., Weyl, 1959; Rutter, 1976, Revil, 1999]. Pressure solution, controlled by gradients of chemical potential differential between stressed-sites and free pore spaces, involves three linked processes of dissolution at the stressed interfaces of grain contacts, diffusive transport of dissolved mass from the interface to the pore space and finally reprecipitation at the less stressed, free-face of the grains [Yasuhara et al. 2003]. This process results in temporally prolonged compaction and the concomitant reduction in bulk porosity and related permeability which has been shown to operate over engineering timescales [Yasuhara et al., 2003].

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