Formation damage caused by fracturing fluid invasion along the fracture face has long been a concern in the petroleum industry. The results of a study on the effect of fracturing fluid cleanup and fracture face damage Oil gas production are presented.
A numerical reservoir simulator is used to simulate (J) fracturing fluid invasion, (2) filtrate cleanup, and (3) gas production after the fracturing treatment. The reservoir properties used in the model are obtained from an example well in the Cadomin formation from the deep basin area of northwestern Alberta. The rock/fluid interaction properties of this apparently "water-sensitive" formation are determined in the laboratory. Results of numerical simulation are used to match the actual pressure buildup test results. The cleanup process of the invaded fluid is demonstrated, as well as how gas production is affected by the permeability damage at the fracture face. An algorithm combining the analytical and numerical methods to determine the fracturing fluid leakoff distribution profile is also summarized.
It is well known in the petroleum industry that formation damage, caused by the invasion of drilling and completion fluids in an unfractured well, can significantly impair theflow of reservoir fluids. When fluid flow from the reservoir into the wellbore is radial, the majority of the pressure losses ccur near the wellbore. Any permeability damage in this region will further increase the pressure drop and greatly reduce the flow rate of the reservoir fluids.
During hydraulic fracture treatments, fracturing fluid or filtrate will leak into the formation along faces of the fracture. This invasion of fluid can reduce the effective permeability to reservoir fluids and impair the flow of reservoir fluids. However, the exposed area of the formation face in a fractured well is much greater than that of an unfractured well. With properly designed fracture conductivity, the pressure loss along the fracture length is negligible. As a result of the large area exposed the flow velocity from the reservoir, through the fracture face, is small. The consequence is a significantly reduced pressure loss around the fracture, compared to that around the wellbore in an unfractured well. Consequently, permeability reduction at the fracture face has a much lesser impact on reservoir fluid production than the permeability reduction around an unfractured wellbore.
The types of fracture face damage may be classified In two categories:
Absolute permeability damage is the reduction in the absolute permeability of the rock within the invaded zone. This may result from polymer invasion, clayswelling or migration, scale or paraffin precipitation, stabilized emulsions, and other similar physical porosity modifications. In this discussion, it is assumed the permanent damage does not change during the cleanup and production periods.
Relative permeability damage is the reduction in the effective permeability to reservoir oil and gas caused by the changes in fluid saturation and rock wettability. This in turn causes the changes in relative permeability and capillary pressure. This type of damage changes as fluid saturation changes.
In 1975, Holditch1 published a classic paper on the factors affecting water blocking and gas flow from hydraulically fractured gas wells. He demonstrated the combined effects of permanent formation permeability damage and relative permeability damage on gas production in low-permeability gas formations. He concluded the damaged zone permeability must be reduced by several orders of magnitude and the capillary pressure altered, before a se