Control of Unconsolidated Sands in Waste-Disposal Wells1
Sand-control methods were first used in water wells, and modified methods later were applied to oil and gas wells. The most recent application for sand control is in waste-disposal wells. The increasing use of unconsolidated sands as disposal zones has created a need for better sand-control systems.
We suggest that the primary causes of sand-control problems in disposal wells are (1) greater completion intervals, (2) intermittent operation of the well, and (3) chemical characteristics of the injected effluent. Therefore, in order to prevent sand production in disposal wells, consideration must be given to (1) formation characteristics, (2) completion fluid, (3) type of completion, and (4) completion method.
Two universally used methods of sand exclusion, with suggested modifications for disposal wells, are the method of in-place sand consolidation with plastics and fhe use of gravel packs in conjunction with sand screens. Sand consolidation has limited application because of the large completion intervals normally used in disposal wells and because of possible chemical reactions with injected effluent. However, a gravel-pack sand-screen completion generally eliminates the three primary causes of sand production in disposal wells. Factors of prime importance are (1) the drilling and completion fluids, (2) formation grain size and composition, (3) size and amount of gravel, (4) pumping rate, (5) pressure, and (6) gravel concentration.
Field and laboratory data show that the method of gravel-pack sand-screen completions can be used successfully over intervals as great as 585 ft (178 m) in unconsolidated Frio sands.
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This publication consists of papers based on oral presentations at a symposium of the same name co-sponsored by the United States Geological Survey and the American Association of Petroleum Geologists. A wide range of technical issues are covered, as well as regulatory and liability concerns. Documentation of two areas in Colorado where earthquakes had resulted from subsurface fluid injection set the stage for modern debates regarding possible similar results elsewhere. A wide range of fluid compositions are subject to subsurface waste disposal. The largest volumes are brines separated during the production of oil and gas wells, but acid-water and industrial wastes of all types can be disposed in significant quantities in local areas. Large hydraulic fracture treatments never recover all of the injected fluids, and the chemical additives in the fluid that remains underground can be a concern. The subsurface injection of radioactive waste is a topic for three of the papers. The possible need for sequestration of carbon dioxide was not a significant concern at the time and was not covered, but many of the papers provide insight into the issues related to modern proposals. When fluids are injected under pressure into subsurface aquifers, they interact in numerous ways. The fluids can potentially migrate for long distances and potentially interfere with other uses for the native aquifer fluids. If the aquifer cannot transport all of the fluids away, the buildup in pressure can cause fracturing of the rock. Differences in composition between the injected and native fluids can cause chemical reactions to occur; in some cases these can be desirable in that they can immobilize certain solutes in mineral form. The long-term environmental consequences are a common theme in many of the papers because of the recognition that the disposed fluids would become a permanent fixture in subsurface aquifers and could have long-term consequences for their future utilization.