Improve industry’s ability to drill and complete gas wells without damage to the permeability of the productive formation. The objective of this project is to accelerate the introduction of new types of non-damaging, drill-in and completion fluids through: (1) development of a kinetic mathematical model to accurately portray the filtercake dissolution process and improve the ability to predict the formation damage behavior of new materials under down hole conditions; and (2) define a new class of drill-in fluids and demonstrate their effectiveness.
Texas Engineering Experiment Station, Texas A& M University (TAMU) – Project management and all research products
College Station, TX 77843
In the early 1990s, industry developed a new class of fluids known as drill-in fluids, to reduce drilling and completion induced damage, especially in horizontal open hole completions. A drill-in fluid (DIF) is defined as a combination drilling and completion fluid, specially formulated to optimize the production capability. Like standard drilling fluids, DIF’s provide lubricity, inhibition, solids suspension, and borehole stability. Additionally, they are also formulated to protect producing intervals by: (1) mechanically sealing exposed pore space openings in boreholes; (2) stabilizing the wellbore during completion; (3) and clean up easily.
Most DIFs contain solid materials. Solids are used as bridging agents to plug the surface of a formation matrix and as weighting material to control formation pressure. DIFs use viscosifiers such as biopolymers to provide gel strength and improve the carrying of the drill solids to surface. Industry experience has demonstrated the solids content of the drill fluid, in particular drill solids, hold the key to fluid performance. If the solids content of the drill fluid could be kept to low concentrations, then open hole completions performed well. If solids control was not maintained, then significant formation damage occurred. A A Texas A&M University (TAMU) project was one of the first academic and industry research program addressing this role of drill solids and identifying ways of predicting the impact of these materials. Laboratory tests were developed to measure the two key factors necessary for determining completion efficiency: (1) filtercake removal and regain permeability; and (2) rate of filtercake removal (breakthrough time).
The project has been instrumental in increasing industry’s awareness of the detrimental role of active drill solids in filtercake cleanup and formation damage. The comprehensive approach of this project in (1) defining the nature of formation damage and (2) identifying steps to avoid it demonstrated how laboratory testing practices can be integrated into field applications. This project also has shown how an investment in laboratory testing can pay off in significantly improved well performance in horizontal open hole well completions.
This project was an extension of an earlier effort sponsored by the Completion Engineering Association (CEA) and begun in 1995 to study formation damage and cleanup techniques used in completions of horizontal, unconsolidated, open hole wells. The project has been supported by 13 different industrial sponsors, and three companies (Conoco, Shell, and TBC Brinadd) have been supporters during the entire project.
During the completion of an openhole horizontal well, metallic screens are lowered into the wellbore while the filtercake developed by the drill-in fluid (DIF) is still on the wellbore wall. This filtercake “sandwiched” between the formation face and the metallic screen can contribute to plugging of the screen, reducing well productivity. Most DIFs contain solid materials: viscosifiers, drill solids, and additives used as bridging agents to prevent lost circulation and as barite weighting material to control pressure formation. During drilling, the filtercake builds up as an accumulation of varying sizes and types of particles. This filtercake must be removed during the initial state of production, either physically or chemically (i.e., via acids, oxidizers, and/or enzymes). The amount and type of drill solids affects the effectiveness of these clean up treatments.
In this project, a laboratory testing program was developed to evaluate the plugging mechanisms for screens after the clean-up of filtercake that formed on an unconsolidated core by two different DIFs. The goal was to measure the change in regained-flow capacity, before and after filtercake clean-up treatment and backflow. The test data show that when filtercakes were removed by backflow, and if these filtercakes were comprised of comparatively smaller-sized particles, the higher the minimum dislodging pressure (MDP) and screen plugging (compared to coarser-sized particles), and the lower the regained-flow capacity. However, if the filtercake were comprised of coarse particle sizes then only a minimum MDP was required, leading to higher regained-flow capacity and less screen plugging. Also, the results indicated that a hydrochloric acid treatment was more effective in removing filtercake than a 3 percent KCl treatment, and that the use of HCl is much more effective in removing a filtercake formed by sized-salt than in removing one formed by sized-calcium carbonate. The laboratory testing showed that the presence of drill solids has a major detrimental effect on cleanup behavior of filtercakes, whether the DIF is sized-salt or sized-polymer carbonate. Drill solids containing clay, because of their small size and active surface area, are harder to clean than those predominately comprised of sand.
Using a systematic approach, TAMU developed a series of mathematical correlations to predict the removal of filtercake deposited by DIFs on formation sands. A database of experimental results was used as the basis for developing empirical models to predict regained permeability and breakthrough time. After performing statistical studies to identify key variables, three independent factors were chosen for each type of DIF to include in the correlation process: drill solids concentration, cleanup fluid concentration, and temperature. This led to predictive models for formation damage and cleanup treatment design.
Additionally, to demonstrate that the guidelines developed from laboratory experiments are valid in field applications, a series of well audits were performed over the course of the project. During this project, the lab results and the results from the field were matched to further develop a set of “best practices.” This resulted in: (1) a complete set of case study audits for field well planning, well construction and well cleanup operations; (2) horizontal well productivity analyses using early-time wellhead production data and wellbore cleanup data; and (3) the direct, scaled correlation of laboratory data to field well productivity.
Finally, a set of guidelines was developed to assist those involved with the construction of high productivity horizontal wells. These guidelines, based on the project’s detailed investigations into the nature of formation damage of commercially available DIFs and completion practices, were designed for use in conjunction with an engineering team’s well design program.
This project has been completed and the final report is listed below under "Additional Information".
Final Report [PDF-7079KB]