Well Completions and Workovers

Well completion is the process of bringing an oil or natural gas well into production after initial drilling or “workover” has been completed. Completion steps involve removal of the drill string, casing, cementing, perforating, hydraulically fracturing in the reservoir to stimulate production, expelling drilling and fracturing fluids, and installing the production valve. Where the completion steps involve hydraulically fracturing in the reservoir to stimulate production, expelling “frac” fluids from the reservoir may result in excessive emissions of methane gas if not captured or flared.   

“Workover” refers to several methods to maintain or restore existing well production, potentially including re-fracturing the reservoir.

Oil and gas wells are drilled in conventional (i.e., porous rock formations) oil and gas reservoirs and in more technologically challenging unconventional gas and oil reservoirs such as tight sands, shale, and coalbeds. Completion of a new well after installing the production valve (i.e., wellhead or Christmas tree) and reworking (i.e., workover) of an existing well in these tight formations typically involves hydraulic fracturing of the reservoir to increase well productivity. Hydraulic fracturing may also be performed to increase productivity in conventional gas and oil reservoirs. 

Hydraulic fracturing is a technique for improving gas and oil production by fracturing the reservoir rock with very high-pressure water containing a proppant (i.e., a solid material, typically sand) that keeps the fractures “propped open” after water pressure is reduced. This process may be carried out in several stages if necessitated by the geometry (vertical or horizontal), depth of the well, and/or the length of laterals (horizontal wells). In a multi-stage fracture, different sections of the wellbore are isolated from the rest of the wellbore and each section is fractured independently.  

Once the hydraulic fracturing step is finished, new and workover gas wells are completed by flowing the well with low density gas at a high rate to lift and expel the water and excess proppant to the surface, clearing the well bore and formation for sustainable long-term gas and oil production rates. This is referred to as the “flowback” stage where drilling and frac fluids are removed from the well bore. Pushing the water and excess proppant vertically up the well with lower density gas results in a mixed flow of gas, water, and proppant at the surface. The long-term production gas/liquid separator is not designed for these high, three-phase (i.e., gas, liquid, and proppant) flow rates. Therefore, it had been common practice during the initial well completion step to flow the well to a pit or tanks where water, hydrocarbon liquids and sand were captured, and slugs of gas vented to the atmosphere or flared.  

Completions can take anywhere from one day to several weeks during which time a substantial amount of gas may be released to the atmosphere or flared. Fractured oil wells do not experience substantial gas bypass and efficiently push the hydraulic fluids up the well which leads to shorter completion times and lower emissions as compared to gas wells. 

Over time, oil and gas wells may require maintenance activities to minimize the decline of production from the well, referred to as “workovers.” Where these maintenance activities include refracturing of the reservoir, many of the same completion steps may result in methane gas venting. 

U.S. Environmental Protection Agency. (2012, April). Oil and natural gas sector: Standards of performance for crude oil and natural gas production, transmission, and distribution – Background supplemental technical support document for the final New Source Performance Standards. https://www.regulations.gov/document/EPA-HQ-OAR-2010-0505-4550