Relationship Between Fracture Propagation Pressure And Pore Pressure
Authors: Salz L.B. 
Published: December 2007

Abstract
Fracture stimulation data have been analyzed in conjunction with reservoir pressure buildup surveys to investigate the relationship between pore pressure and fracture propagation pressure in the geopressured Vicksburg formation in South Texas. The empirical relationship derived from the data can be used to improve well control and planning during drilling or workover and aid in stimulation design in partially depleted geopressured zones. Although limited to a specific formation and geographic area, application to other geopressured provinces may be made if substantiated by field data.

Introduction
Fracture stimulation in the McAllen Ranch, Javelina and McCook fields in Hidalgo and Starr Counties, Tex. (Fig. 1) is required to improve production deliverability and ultimate recovery in the geopressured Vicksburg formation. Instantaneous shut-in pressure data obtained during 75 fracture treatments performed on originally pressured and partially depleted reservoirs were analyzed to provide some insight into the reduction of fracture propagation pressure with decrease in reservoir pore pressure. Included in this study are six completion intervals fracture treated initially at original reservoir pressures and refractured years later at significantly reduced reservoir pressures.
Pore pressure and fracture propagation gradients examined r e from 0.197 to 0.955 psi/ft and 0.623 to 1.024 psi/ft, respectively. Pressure gradients were used to incorporate pressure and depth variables into a single, commonly used term. Fracture initiation pressures have not been included in the investigation because of the complexity and uncertainty of analyzing these pressures from surface pressure measurements.

GEOLOGY
The Oligocene-Vicksburg sand-shale sequence of South Texas has a relative age of post-Eocene Jackson and pre-Oligocene Frio. Average thickness of the Vicksburg along the McAllen Ranch-Javelina trend is some 7,000 ft and underlies portions of seven counties. Depth range along the trend is 7,000 to 15,000 ft. Both oil and gas are produced, but a greater proportion of production is geopressured gas-condensate from the lower Vicksburg.
Environment of deposition varies from continental to inner neritic, with the bulk of the numerous pay sands being deltaic (Fig. 2). Structured interpretation is highly complicated by uncorformities, intricate fault patterns, erratic deltaic sedimentation and secondary cementation. Structures vary from fault-anticlines to upthrown fault closures.
Productive intervals included herein range from the McAllen Ranch "N" sand at 9,960 ft to the McAllen Ranch 15,300-ft sand. All sands are characterized by abnormally high initial pore pressures with observed pressure gradients as high as 0.955 psi/ft. Due to the deltaic depositional environment of the rock the sand distribution is erratic, often resulting in only a fraction of the gross thickness being reservoir quality rock. Thus is illustrated by Fig. 3, which presents a log profile of two pay sands, the "P" and "S" sands, in Mckllen Ranch field.

ROCK PROPERTIES
The Vicksburg formation is a tight, low-porosity, low-permeability sandstone formation composed mainly of quartz, feldspar, volcanic fragments, and cementing material. Reservoir rocks are typically very fine-grained sandstones with average porosity of 12 to 18 percent and permeability of less than 1 md. Table 1 presents average rock composition and rock properties.
Although a competent sandstone, formation collapse has occurred in some completion intervals as a result of high drawdown pressures and/or high production rates. Removal of the calcite cementing material by acidizing enhances formation collapse by significantly reducing rock strength.