Novel Coiled-Tubing Perforation Technique and Stimulation in Carbonate Gas Well
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Abrasive perforating with coiled tubing (CT) is a technique that has proved to be a valuable alternative to conventional perforating with electric line. The application is particularly valuable whenever a high rate or fracture-stimulation treatment is to follow because of the significant reduction in tortuosity and pumping friction losses across perforations. This paper discusses a novel approach to abrasive perforating, including the first-ever use of an acid-soluble abrasive material and ending with CT-jetting-assisted nitrified stimulation.
Many carbonate gas wells located in the northern section of the Ghawar reservoir exhibit a low-permeability profile and are characterized by having low reservoir pressure. Fracture gradients in a formation in this area of the field are on the order of 1.12 psi/ft, which is considerably greater than that of more-conventional tight gas formations. One particular well, located on the flank of the reservoir, was designed to be completed with a plug-and-perforate acid-fracturing operation, but issues arose with this option that finally caused the abandonment of the fracturing treatment. To find a solution that would be feasible and economically viable, an innovative perforation and stimulation technique was engineered for this well.
Design of the Perforation Technique and Yard Testing
The team was able to source a newly developed acid-soluble abrasive material that enabled the same quality of perforations to be created without performing a cleanout run because the acid from the stimulation would dissolve the abrasive material, saving at least 2 days of operation.
The abrasive material was sourced, and several yard tests were completed to determine if the material had the same abrasive characteristics as common 20/40- or 100-mesh sand. It was necessary to compare the time required to penetrate the steel and cement with the results obtained using common sand. This benchmarking was key to determining if additional time and fluid resources were necessary to obtain the same penetration.
A yard test was set up with two metallic 55-gal drums cut halfway lengthwise and then welded together. This created a receptacle that allowed the team to place a 6⅝-in. casing section with 100% standoff and then fill the entire annular space with cement. The drum was then set on top of cement blocks, and the abrasive-jetting tool was introduced.
The tests were performed on several drums to establish a control test with the drum by use of 20/40-mesh sand and the same tool configuration and then using the same test setup to pump the acid-soluble abrasive material. Conventional sand was able to cut the pipe and penetrate the cement completely through the 55-gal drums after 3.2 minutes of pumping. Meanwhile, the acid-soluble material required 6.8 minutes to penetrate the cement and ultimately cracked the cement annulus (Fig. 1). Although the cement annulus cracked at 6.8 minutes, water was observed exiting the sides of the steel drums at approximately 4.8 minutes. These tests helped provide a better understanding of the additional pumping time necessary to achieve the same penetration effect with the new product compared with the conventional sand.
After the tests were completed, the design of the operation was begun. It was determined that a total of six perforation clusters would be performed. Each of these clusters would include five perforation stages with four holes per stage, for a total of 120 perforations. As such, it was determined that the pumping time to achieve a similar effect would be 45% longer compared with that using conventional sand; it was thus decided to increase the value to 12 minutes (roughly 46% additional time), for operational ease.
Design of the Stimulation Operation
This critical portion of the operation required the use of a stimulation technique that would maintain a wellhead pressure (WHP) of less than 10,000 psi but would provide deep acid penetration at the same time. The decision was to use the CT-jetting-assisted acid-fracturing approach to meet both criteria, leveraging the Bernoulli effect of dynamic diversion and deep penetration by the use of CT. This is achieved through the use of CT-jetting-assisted acid injection, which takes advantage of the dynamics of fluid moving at a very high velocity to direct flow to a specific entry point in front of the nozzle.
The pumping schedule for the CT-jetting and stimulation stages was then developed. The first two stages of the first cluster were planned as a diagnostic approach to determine whether the acid-soluble abrasive material could contact the formation as desired; if not successful, switching to conventional sand would be required. If successful injectivity was observed, then the rest of the stages would be completed, with the stimulation performed sequentially immediately after each CT-jetting stage.
The equipment was rigged up per operator requirements. A drifting and casing-collar-locator run was performed with a memory gauge to achieve precise depth correlation for the perforation and stimulation stages. The run was completed successfully, and the depth offset was corrected with the flags made on the pipe. CT was run in hole (RIH) once more with the perforating/jetting tool to perform the first two stages. The plan was to perform the first two stages and then to perform an injectivity test to verify the effectiveness of the abrasive material. The injectivity test showed a steady pressure increase. The contingency plan was then to spot a 50-bbl 10% hydrochloric acid pill to increase injectivity in case the face of the formation had been plugged. The CT was pulled out of hole (POOH) 2,000 ft to stay above the acid after being spotted. At this depth, the acid was squeezed into the formation, and the pressure suddenly dropped to zero psi while being squeezed. No pressure was observed at the surface. It was determined that the well was on vacuum from this point forward.
It was decided to proceed with a nitrified CT-jetting-assisted acid injection, which would still produce fractures, although with less extension. It was decided that the new pump rates would be 5 bbl/min or higher—to maintain CT pressure of greater than 6,000 psi to provide control of fluid placement—with a nitrogen rate of 600 scf/min. The entire treatment was pumped with these parameters, and then CT was POOH. The increase in pressure observed while performing the CT-jetting operation corresponds to the additional pressure in the reel while it was being loaded with a higher-density fluid, the abrasive material, which then dropped as the slurry entered into the section of CT in the well. Eventually, the pressure increased when the acid entered the reel and eventually hit the formation, but then was reduced once the acid dissolved the near-wellbore damage in the face of the perforation.
External downhole pressure gauges were run to evaluate the treatment later. In this case, it was observed that the annulus pressure was in the vicinity of 5,200 psi, which is in the same range as a full column of water in the annulus, indicating the positive effect of nitrifying the acid treatment to maintain the acid placement under control.
To evaluate the result of the treatment, it was necessary to use a wellbore simulator to evaluate the effectiveness of the perforations that were made. The stimulation treatment that followed the perforations was evaluated with a reservoir simulator.
The results of the simulation showed that the fractures created were 55 ft long and approximately 50 ft in height, with an etched width of approximately 0.3 in. The simulated bottomhole pressure (BHP) closely matches the synthesized BHP in that portion of the flowback after the choke size has been left stable, which is the portion of the flowback period that is useful for correlating and evaluating, given that the other pressures are at rates that have not been stabilized and the choke setting is constantly varied.
The treatment was completed successfully, and the CT was used to perform a nitrogen-kickoff operation to bring the well into production. CT was RIH while unloading the well. The deeper the CT was RIH, the higher the circulation pressure, evidence that the nitrogen had to lift a continuous liquid column in the annulus. CT was downhole for 15 hours at 9,000 ft while pumping nitrogen at 500 scf/min. Two batches of liquid CT protector fluid were pumped to protect the CT from reservoir fluids during flowback. No circulating pressure buildup was noticed; however, WHP increased significantly from 2,200 to 3,000 psi.
After a constant gas rate was obtained, the CT was demobilized from the location and the flowback continued to clean up the treatment. Eventually, the well stabilized at approximately 30 MMscf/D, which is an outstanding figure, especially for a well that was subjected to a “last resort” treatment before being sidetracked because of wellbore issues.
The operation was performed successfully. The acid-soluble abrasive material was deemed a total success, given that the perforations were generated in their entirety, and allowed for the full acid treatment to be completed. The placement of the perforations with regard to the target pay zone and the subsequent acid treatment paid great dividends; the final flowback results came in six times higher than expected.
Novel Coiled-Tubing Perforation Technique and Stimulation in Carbonate Gas Well
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