Abnormal Treating Pressures in Massive Hydraulic Fracturing Treatments
Authors: Medlin W.L. , Fitch J.L. 
Published: December 2007

Abnormal treating pressures were observed during massive hydraulic fracturing (MHF) treatments in the Mesa Verde formation of the Piceance basin, CO. Data from three widely separated wells and in several zones per well showed a pressure increase during MHF treatments that we call "pressure growth." This pressure growth was at least semipermanent. The elevated instantaneous shut-in pressures (ISIP's) did not return to initial values over periods of pressures (ISIP's) did not return to initial values over periods of several days.

The magnitude of this pressure growth is highly variable. When its value is less than about 2,300 psi [15.9 MPa], the MHF treatments are usually completed and results are obtained that are within normal expectations. When its value exceeds 2,300 psi [15.9 MPa), sandout occurs and the fracture length estimated from production data is much less than that calculated with crack production data is much less than that calculated with crack propagation models. Temperature logs indicate little or only propagation models. Temperature logs indicate little or only modest vertical extension of the fractures above the perforations. These data, along with sandouts, point to a large increase in fracture width in response to pressure growth.

One possible cause of pressure growth is fracture branching. A multiplicity of branches could produce a plastic-like effect. Laboratory measurements have ruled out plasticity as the cause. The stress/strain behavior of the rock is similar to that of rocks where no pressure growth occurs.

Pressure growth seems to depend on both pumping rate and Pressure growth seems to depend on both pumping rate and fluid viscosity. Thus, there is some hope for its mitigation through treatment design. Also, pressure growth appears to correlate negatively with pay-zone quality. This suggests that the phenomenon can be exploited as a fluid-diversion technique. phenomenon can be exploited as a fluid-diversion technique.

Introduction

Because of its large resource, the Piceance basin has been one of the more promising prospects for MHF applications in tight-gas sands. In this promising prospects for MHF applications in tight-gas sands. In this basin, the Mesa Verde and adjacent overlying formations provide several hundred feet of sand thickness at depths between 5,000 and 12,000 ft [1525 and 3650 m]. Porosity in much of this sand thickness ranges from 5 to 8%. The gas resource in the basin has been estimated at 33 Bcf [934 × 10(6) m3]. Massive fracturing methods have been tested extensively within the Piceance basin by Mobil and others. Results of these tests have shown that, with market conditions of earlier times, commercial wells can be developed in the Piceance basin using massive fracturing methods.

MHF experience in these tight-gas sands shows an unusual characteristic. Fracturing treatments are always accompanied by large increases in treating pressure. This phenomenon, or pressure growth, has adverse effects on pressure. This phenomenon, or pressure growth, has adverse effects on fracture effectiveness. It appears to produce undesirable width/length aspect ratios as judged by fracture height and production data. It limits the size of massive fracturing treatments by causing premature sandout. And it increases the pumping horsepower requirements by as much as a factor of two.

Understanding and finding ways to avoid this problem are clearly matters of practical importance. Pressure growth inhibits the generation of very long practical importance. Pressure growth inhibits the generation of very long fractures, which are needed for successful application of MHF methods in the Piceance basin. It is likely to be important in other basins where tight-gas sands are lenticular on the scale of Mesa Verde lenses in the Piceance basin.

Pressure growth is not to be confused with any of the pressure changes treated by Nolte and Smith in their analysis of fracturing pressures. The pressure-growth phenomenon considered here begins with the first injection pressure-growth phenomenon considered here begins with the first injection of fluid into the fracture and continues throughout the treatment. It produces dramatic effects, typically resulting in a doubling of the surface produces dramatic effects, typically resulting in a doubling of the surface treating pressure in a massive fracturing treatment.

In this paper, we present field data that help define the nature of pressure growth. We present a model based on fracture branching that may pressure growth. We present a model based on fracture branching that may explain the cause of pressure growth and is consistent with field observations. Other explanations should also be investigated. We suggest a possible way of avoiding or minimizing the problem and propose a method of using the pressure-growth phenomenon as a fluid-diversion technique.