Abstract

This field study was based on 50 fracturing jobs on 42 wells that Halliburton pumped from 1977 to 2006 for Petro Canada Ltd., producing from the Viking formation in Ricinus area. Four different fluid systems (guar/borate, CO2, gelled oil and gelled CO2, oil) were investigated to compare their effects on the skin factor, effective fracture half-length, percentage of screenout jobs, and production rate. All available fracturing and buildup data were collected and analyzed. Gelled CO2 /oil fracture treatment showed the best results on the effective fracture halflength and production rate that proved to be the best option among these four systems. This study was also focused on the investigation of the reasons that a high percentage of fracture jobs have screened out; 85% of the fractures investigated exhibited pressuredependent leakoff (PDL) behaviors believed to be the primary characteristic of the unsuccessful jobs.

Treating the formation at the pressure lower than the fissure opening pressure was considered to be a possible way to avoid screening out. A field case was used to investigate this possibility with the application of a fracture-simulation model, but it was not possible to pump at a rate low enough to prevent opening the natural fractures Since July 17, 2002, the service company fractured all wells with gelled CO2/oil at high pumping rates and achieved 100% of successful rate. This proved that pumping at a high rate is only option so far to combat PDL and place the proppant successfully. Economic value to the customer (EVC) analysis showed $460,000 savings on eight gelled CO2/oil high-rate fracture jobs for Petro Canada Oil and Gas Ltd.

Four fluid systems have been used to pump 50 fracturestimulation jobs in 42 Ricinus-area wells producing from the Viking formation. Fig 1: shows the area of these 42 wells. Among these 50 fracturing jobs, 6 used guar/borate, 12 gelled oil, 15 CO2, and 17 gelled CO2/oil jobs have been pumped since 1977. Fig. 2: shows the percentage distribution for the four systems involved in this study. The following lists brief descriptions for the four fluids:

  1. Guar/borate. This is a borate-crosslinked guar gum or hydroxypropyl (HP) guar water-based fracturing fluid that provides high viscosity at low cost. Guar/borate fluids produce a rapid crosslink that reheals after shearing. The disadvantage of this fluid is that it requires precise pH control and rapid crosslinking reaction, and generates greater friction pressure.

  2. Gelled oil. This fluid is a continuous mix fluid formed using an iron crosslinked phosphate ester surfactant. The use of surfactant chemistry helps preclude damage resulting from polymer residue. There are many advantages to this system, such as high temperature application up to 135 °C, easy field operations, superior proppant transport and leakoff control, in combination with high regained permeability and fracture conductivity.1

  3. Gelled CO2/oil. 2 This system is ferric iron-crosslinked, phosphate ester gellant for miscible mixtures of oil and CO2; it is designed for continuous-mix stimulation ofgas reservoirs over a wide temperature range.

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