Injection of Oilfield Produced Water into Water Aquifers - The Ojo Alamo Project
Authors: Russell C.S. , Hazlett W.G. 
Published: December 2007

Abstract
This paper shows the feasibility of using the Ojo Alamo Sandstone (OAS) aquifer of the San Juan Basin (SJB) to store produced water from oil and gas operations. Included are data analysis and reservoir modeling. To perform this study, we have constructed a reservoir (aquifer) simulation model that allows us to estimate the effect of injecting low salinity produced water from oil and gas wells into the Ojo Alamo Sandstone. Using the simulation model, we have the ability to select sites for both the injection and discharge of water within the reservoir boundaries and predict water quality at any point in the aquifer. The OAS's insitu water and formation properties were compiled from recent studies. In areas of the OAS which have insufficient data, electric log correlations from oil and gas wells were utilized to increase the accuracy of the model. The injected water quality is based on current production water analyses available from area operators. Realistic variations of injected water quality and flow rates are simulated in the model to create potential water quality distributions within the reservoir. The short range benefits of this project will be to provide readily available storage for production water for later use in agricultural and urban areas. Certain low salinity production water that is currently considered a "waste product" could be turned into a valuable resource. A benefit for New Mexico is more usable water and less unused "waste water".

Introduction
The San Juan Basin of southeast Colorado and northwest New Mexico produced approximately 56 million barrels (6.5 MGD) of water in association with oil and gas production in 1994. Of this amount, over half was produced from the Fruitland Coal Formation. The Fruitland Coal is considered an aquifer in the northern basin where the water held may contain less than 300 ppm total dissolved solids (TDS). In the south, the water becomes brackish and averages near 10,000 ppm TDS (Table 1). Water produced in conjunction with natural gas production often has negligible associated hydrocarbons. As per the prime directive of the Resource Conservation and Recovery Act, this water should be made available for future use. Presently, it is being pumped into deep, relatively tight formations where the insitu water is often much poorer than the injected water. A possible alternative to this course of action is to pump this water into relatively shallow formations that are fully saturated, but contain less than desirable water.
This investigation studied the Ojo Alamo Aquifer as a possible receptacle for this water. This aquifer is found across the San Juan Basin containing water, for the most part, that is unsuitable for drinking. Only the southernmost region of the OAS is now being used domestically. Fig. 1 shows the electrical conductivity (EC) of the Ojo Alamo water. Here, the outcrop is noted by the heavier line and is dashed where the boundary is uncertain. This study found that both the injected and the OAS water's TDS in ppm were equivalent to the EC in micromhos/cm. By and large, the aquifer is frIlly saturated and possesses excellent hydrogeologic properties. Formations that bound this aquifer contain multiple shale layers with insitu water that usually has higher TDS concentrations than the OAS water.
Produced water quality in the San Juan Basin varies. Table 1 lists the chemical components in a central basin Fruitland Coal well - Coal wells in the north will be at least 4 times as fresh. Findings herein show that concentration changes in the aquifer by the injection of this type of water are minimal due to the vast pore space volume contained by the Ojo Alamo.
It is proposed that the natural gas operators be allowed to inject their production water into the OAS on a case by case basis. Approval of such applications could be granted upon complete investigation of an area larger than the expected area of intrusion. Operators currently allow for charges of $0.30 - $ 1.00 per barrel disposed. Injection into the OAS could conceivably be less than half this cost.
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