Abstract

Productivity of a tight gas formation is strongly tied to flow area. This fact dictates the creation of enhanced flow area through drilling of multi-lateral wells and/or fracturing.

In the case of fracturing, loss of fracture conductivity near the wellbore can have a drastic and negative effect on the productivity of a fractured well. This is especially true when creating a transverse fracture to intersect a horizontal well. In tight formations, loss of communication between the fracture and the formation could also have a devastating effect on productivity of the well, vertical or horizontal.

In this paper, the following will be discussed:

  • The effect of completion methodology on productivity of a tight gas formation

  • The effect of near-wellbore damage of fracture conductivity on productivity of a fractured well in a tight gas formation

  • The effect of creating multiple fractures on the productivity of wells

  • Testing a previously fractured well using an injection falloff test, or a pressure buildup test to determine the degree of damage (DED) and depth of damage (DOD) of the fracture conductivity

Introduction

Tight hydrocarbon bearing formations are usually fractured to accelerate production. Fracture length and conductivity are designed to optimize the well productivity. The designed fracture parameters are length, height, and conductivity. Many papers and techniques have been developed to design optimum fracture conductivity. Many other techniques have been developed to calculate the fracture parameters from various tests. These tests may be pressure buildup or drawdown, DST, and other well-testing techniques. History matching is another approach used to estimate the fracture parameters from production data. Several minifrac analysis techniques have been developed to also calculate fracture parameters.

Uniform fracture conductivity is usually assumed, however some authors have examined the effect-changing conductivity inside the fracture (Sneddon 1946). Soliman(Soliman et al. 1987) pointed out that loss of fracture conductivity near the wellbore could have a drastic and negative effect on the productivity of a fractured well. This is especially true in the case of creating a transverse fracture to intersect a horizontal well.

Most fractures do not have a uniform conductivity. Actually, uniform fracture conductivity is not a necessity. It has been shown (Soliman 1986) that a fracture with declining fracture conductivity away from the wellbore may behave in a manner similar to that of one with uniform fracture conductivity. This makes it necessary to maintain good fracture conductivity near the wellbore. On the other hand, loss of conductivity near the wellbore reduces this apparent fracture conductivity.

Loss of fracture conductivity near the wellbore may sometimes occur as a result of the fracturing process. Some of the reasons leading to this include:

  • Degradation of fracture conductivity over time caused by weakening of proppant

  • Loss of fracture conductivity because of an increase in closure pressure resulting from formation depletion. The failure of proppant may be accelerated by the cyclic change in pressure caused by periodic shut-in.

  • Limited number of perforations

  • Fracture tortuosity

  • Overdisplacement of the proppant, leaving the fracture with virtually no proppant in the vicinity of the wellbore.

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