Improved Drilling Efficiencies Using Offline Cementing Saves Time, Reduces Costs
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Drilling in the Appalachian Basin in Pennsylvania has evolved since its inception. Operators have shifted their focus from mere wellbore delivery to delivering wells in the shortest amount of time to reduce risks and costs and drive efficiency. The complete paper presents a case study in which offline cementing improved operational efficiency by reducing drilling times and provided significant cost savings.
For decades, the focus during drilling operations has been on achieving faster rate of penetration (ROP) and drilling time to reduce well costs. However, the significance of ROP improvement accounts for less than 25% of the full well-construction cycle. Many areas with possible efficiency gains have yet to be explored within the cycle.
When an operator began making plans to switch from traditional drilling (top-down drilling of surface through production sections, per well) to batch-set drilling (drilling all surface sections, followed by all intermediate sections, then all production sections), the service company proposed the offline cementing method as a cost-saving and efficiency-conscious measure.
Theory and Definitions
The concept of offline cementing itself is quite simple: The openhole section is drilled, and casing is run to total measured depth and hung in the wellhead. Then, the rig skids to the next well, begins rigging up, and drills the next openhole section. While the rig is preparing on the next well, the cement-service provider can perform operations on the previously drilled or cased well section.
Risks with this type of operation were assessed during pre-job planning. Multiple risk assessments were conducted on location before the onset of operations, and a job-site analysis (JSA) was also completed.
Pennsylvania state regulations dictate that, on surface and intermediate strings, cement must reach a compressive strength of 350 psi and a minimum static time of 8 hours before moving forward with operations after the primary operation. Because of this wait time, offline cementing is ideal. Operators can skid the rig, leaving the cemented pipe to cure while rigging up on the next well. This helps increase efficiency by minimizing the nonproductive time (NPT) incurred during the 8-hour wait time.
Equipment and Process Description and Application
During the project, the operator used a 24×18⅝×13⅜×9⅝×5½-in. casing design. The wellhead was a multibowl RSH-2N wellhead. No modifications to the wellhead design were necessary to perform cementing operations offline. Casing was landed out on a load ring, eliminating the need to have a precut landing joint on location, which provided additional cost savings to the operator. A landing adapter (the red connection in Fig. 1) was used to stab into the wellhead to accept the plug container.
For offline cementing, the surface equipment is essentially the same. However, some considerations should be accounted for when rigging up the cement equipment. Following the risk assessment and JSA, the pressure necessary to lift was noted to be close to the pressure limits of the equipment. Therefore, the decision was made to chain down the cement head. To help ensure further safety, the cement head was chained to load rings welded to the conductor casing. The plug container used on location for the primary cementing operations included an integral quick-latch coupler, which enabled personnel on location to rig up quickly but safely.
The landing joint must be as close to ground level as possible to help ensure the safety of personnel. The objective is to be able to rig up the cement head without the assistance of a ladder or fall protection.
Because offline cementing was only performed on the surface and intermediate strings, returns were managed straight into the cellar. Sump pumps were used to transfer the returns to open top tanks for disposal.
Defining the red zone is important to any successful operation on location. The red zone is the area outside of which all personnel on location should remain when there is pressure on the cementing iron lines. Conventionally, the red zone is a single path along the rig or up to the rig floor, sometimes zoning off one side of the rig and rig floor. During offline cementing, depending on the direction in which the rig will be moving, the red zone can extend in area, and in some cases, across areas that are a common pathway for the rig crews to continue their operations on the next well on the pad.
Static time between landing the casing and cementing rig up is crucial, particularly if an oil-based, synthetic-based, or diesel-based drilling fluid is present in the wellbore. As these fluids build gel strength over time, the pressure required to break circulation can be affected, which ultimately will affect the displacement efficiency of the cementing program. Proper wellbore conditioning should be performed before the cementing operation to improve displacement efficiency and cementing success. Because offline cementing was performed on the top hole and air-drilled strings, static time was not as crucial. The openhole section was left unloaded until the cementing personnel arrived on location. The service company loaded the hole with fresh water, broke circulation, and proceeded with the cementing operation as planned.
After evaluating the gas flow potential (FP), a low-Portland-cement design with enhanced mechanical properties for the life of the well was selected to assist in mitigating shallow gas migration.
The FP is a calculated value used to determine whether a fluid will be able to flow from the reservoir through the cement annular column. A post-set expansion additive was also introduced into the blend because of the drilling area’s susceptibility to high gas FP. A computer simulation was conducted to calculate the FP for the intermediate section (Fig. 2). Ideally, anything in the moderate zone should be considered when designing the cement blend.
Offline cementing helped save significant rig time during the drilling operation; skidding between surface strings and skidding between the intermediate strings saved 15 and 16 hours, respectively. This helped the operator realize an $80,000 savings. Additional hours were minimized because an estimated call-out time for the cementing operation is no longer necessary. Once casing is landed, the cementing service provider can begin cementing the previously drilled or cased well section.
- Continuously improving the service company’s performance on location ultimately reduces the exposure of personnel and contributes to decreasing well costs. Using an offline cementing program, the operator is able to determine better when cementing personnel can perform primary operation. This helps reduce NPT and additional hours necessary to perform the primary operation, which are the main contributors to increased cementing costs. Further considerations are being made to implement offline cementing on production casings.
- Cement was fully circulated to surface in accordance with the design of service and state regulations on all strings. Zero health, safety, and environment incidents occurred during execution, and zero NPT was incurred throughout the operation.
- Offline cementing resulted in a savings of $80,000 per well.
Improved Drilling Efficiencies Using Offline Cementing Saves Time, Reduces Costs
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