Quantifying Productivity Loss and Permeability Alteration Due to Formation Compaction During Frac-Pac Treatment

Abstract

The objective in Hydraulic Fracturing is to increase well productivity and enhance or accelerate hydrocarbon recovery. Yet in soft formations, where hydraulic fracturing provides the secondary benefit of sand control, FracPack operations may result in adverse effects. These effects are more prominent in Gulf of Mexico (GOM) deepwater fields, where sands are over-pressured and highly compactive. Porosity loss around the fractured well normally occurs during FracPacks. This paper addresses this issue and quantifies the permeability loss in FracPack treatments.

Forces or pressures applied on soft rock surfaces, like those occurring during a FracPack operation, generally lead to compaction (plastic deformation). Here, we exclude Water Pack and High-Leak-off treatments. These deformations are usually accompagned by higher treatment net pressures and wider fracture widths. In recent publications (1-2), fracture stiffness and propagation were addressed during FracPacks. Considerations of both factors provide new design basis for proppants and injection schedules to optimize treatments.

The current paper addresses the leak-off and compaction responses during FracPack jobs and post-fracture productivity. The fracturing process in soft formations can be explained as expansion of the formation at the fracture tip (propagation) and a compaction of the formation on either side of the fracture face (stiffness). The compaction and stiffening of the rock at the fracture sides have great influence when evaluating fluid flow response (leak-off or production). The compaction occurs at the expense of pore space/porosity reduction around the surfaces of the fracture and creation of an extended damage zone into the formation. Permeability damage and reduction in the productivity or injectivity index follows as a result of the compaction effects.

The reported work quantifies the three basic unknowns in this problem: 1) The extent of the compaction zone and what are the dominant controlling parameters. 2) How much reduction in porosity can be expected and how is it distributed within the compaction zone. 3) The expected impact on leak-off and productivity as a result of reduction in porosity and the corresponding permeability. Existing GOM field data relating permeability and porosity/compaction will also be discussed.

Introduction

Frac-Pack completions are aimed at improving productivity in soft formations by preventing sand influx and providing a high conductivity channel for fluid flow. Such formations are typical in the Gulf of Mexico, where these completions were first introduced in the early 1990's. The completion process is characterized by performing a tip screen-out hydraulic fracture, followed by gravel pack placement.