Abstract
A vast quantity of produced water (3.8 million barrels in 2002) is generated during gas production in the San Juan Basin, New Mexico. Most of the saline produced water is re-injected into disposal wells at an average cost of $1-$4 per barrel. Due to the persistent drought condition in New Mexico, beneficial use of the produced water is receiving increasingly more attention in the gas production industry. Based upon the Resource Management Plan (2003) proposed by the Bureau of Land Management, about 10,000 new wells will be approved on federal land in the San Juan Basin for the next 20 years. Given the volume of fresh water required for hydraulic fracturing during well development, reclaiming produced water as the base fluid for fracturing not only may alleviate the impact of fresh water shortage, it also may lower the cost of well stimulation treatment.
This project aims at 1) identifying the critical parameters and the corresponding optimal ranges in the laboratory, which will render produced water applicable as the base fluid for crosslinked gel-based hydraulic fracturing and 2) evaluating the validity of the laboratory findings under actual stimulation conditions in the field.
Produced water from a gas-producing well near Aztec, New Mexico was used as the base fluid for the bench-scale experiments. Crosslinked gels were formulated with carboxymethyl hydroxypropyl guar gum (CMHPG), a zirconium-based crosslinker, persulfate breakers, and other ingredients. The apparent viscosity of the gels under different shear rates was measured using Fann Model 35A viscometers and the bottom-hole temperature simulated in the experiments was about 128 °F. The rheological effects of several parameters, including pH, hydration time, ionic strength, and presence of sulfate, were evaluated. The experimental results demonstrated the feasibility of using untreated produced water from Aztec, NM as a base fluid for crosslinked gel-based hydraulic fracturing. Hydration time and pH were shown to influence the maximum apparent viscosity significantly in the absence of breakers. A positive correlation was observed between the maximum viscosity and the hydration time. For the solution pH, gellation at pH 5.5 exhibited the best rheological performance. By adjusting the hydration time, the gel loading, and the amount of breakers applied, it is conceivable that crosslinked gels with optimal rheological characteristics can be formulated with untreated produced water as the base fluid for well stimulations.
A field test was performed following the bench-scale experiments to validate the laboratory findings under the actual stimulation conditions. A well located in San Juan County, New Mexico was restimulated using 70Q CO2 foamed 30-lb crosslinked gel. Untreated produced water was used as the base fluid for the crosslinked CMHPG gel. Natural gas production averaged 50 thousand standard cubic feet per day (MCFPD) prior to the stimulation and was increased to an average of 100 MCFPD after treatment indicating that the fracturing treatment using untreated produced water was successful in increasing the formation productivity.
