Abstract

The poorer-than-expected performance of some fracturing treatments has been an issue for decades. Considerable effort has been devoted to improved modeling of fracturing treatments so that improved expectations can be provided. Fracturing fluids have been modified to enhance cleanup. Proppant conductivity studies have lead to a better understanding of fracture performance. Yet, there are still many treatments, particularly in low-permeability gas wells, that defy efforts to clean up quickly and to produce at the expected rates. This paper revisits the question of whether fracture face damage is an issue in the subsequent performance of a gas well. It will be demonstrated that the landmark paper by Holditch[1] has been misquoted for 25 years. A numerical simulator has recently been written that has reproduced the earlier work, but also expands on it by demonstrating the physical mechanisms by which fracture face damage can reduce gas production and accelerate water production. The simulator includes relative permeability curves for both gas and water, and capillary pressure functions. The role of Laplace pressure, or capillary pressure, will be highlighted in the explanation of how fracture face damage can cause significant loss in well productivity. In addition, the role of relative permeability to gas will be highlighted as to how it ultimately leads to decreased gas production and increased water production when the fracture face is damaged.

Introduction

The issue of whether fracturing fluids damage the productivity of propped fractured wells is easily 50 years old.[2] Yet, even today our industry is concerned with the causes and remedies for the slow cleanup of fracturing treatments in low-permeability gas wells. Numerical modeling has been a mainstay of the efforts to understand the processes that occur in the formation during and after a fracturing treatment.[3–6] The landmark paper by Holditch in 1979 that numerically explored the effects of formation damage in the matrix extending from the fracture face seemed to settle the issue for most. For most readers, the paper seemed to say that fracture face damage was not an issue, and that formation damage could extend 6 inches from the fracture face with a 99.9% permeability loss before any effect on productivity would be noticed. However, quoting directly from that paper: "... it is obvious that the combined effects of damage, relative gas permeability reduction, and an increase in capillary pressure in the damaged zone can cause severe reduction in gas productivity." Holditch subsequently reiterated this point in various fashion over the next several years as he continued to explore the causes of poor performance with field data and numerical studies.[7,8] Yet, somehow the notion persisted that fracture face damage did not matter.

The fact is that the original work essentially consisted of three numerical experiments. The first experiment included gas production with a damaged zone using a single-phase simulator. This was a set of control experiments, or control calculations. These calculations should not be confused with a gas well with 10% water saturation that produces no water. In this first case of a single phase, it is numerically true, and we have verified it with the model presented here, that the permeability in the damaged zone would need to be reduced by at least 99.9% before a loss in gas production could be observed.

The second experiment was to explore gas productivity with a two-phase simulator, but without any damage. Once again, this was a set of control calculations. The numerical experiment explored whether gas production could be restored once fracturing fluid invaded the matrix next to the fracture face. The two primary issues were:

  1. whether production drawdown can overcome capillary pressure and the discontinuity at the fracture face, and

  2. whether the fracturing fluid can be imbibed deeper into the matrix.

Either one of these processes would be sufficient to restore the relative permeability to gas, and thereby allow gas production. These calculations are relevant to gas wells that have been fractured but not damaged. In this second case, it was reported that as long as either (1) the water was mobile and could be imbibed, or (2) production drawdown could overcome capillary pressure, then water invasion into the fracture face would not affect gas production.

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