Drilling Optimization Improves Drilling Performance and Safety While Reducing NPT
Drilling costs are a critical factor in determining the financial returns from an oil and gas investment. Nonproductive time (NPT) accounts for approximately 20% of all rig time and can be much higher in difficult fields. Drilling optimization is the key to improving drilling efficiency, safety, and reducing drilling-related NPT events, such as stuck pipe, MWD/BHA failures, lost circulation, hole cleaning, and wellbore stability—even small reductions in NPT can result in major cost savings.
Sperry Drilling’s ADT® drilling optimization service combines experienced personnel, modeling, data measurement, and software techniques to optimize drilling performance through drillstring integrity, hydraulics management, and wellbore integrity. Data are acquired through downhole, MWD, and surface sensors, and managed with the InSite® rigsite information system.
Drilling-optimization programs that use fit-for-purpose tools and knowledgeable and experienced personnel can produce significant improvements in drilling performance, safety, and NPT, at little additional cost.
Downhole vibration is a complex and costly problem that often results in failures of MWD tools and BHA components, poor drilling efficiency and high NPT.
After an operator experienced a number of vibration-related component failures in a Gulf of Thailand well, including MWD and BHA components, a study was undertaken to determine the mechanisms responsible for these failures. The target reservoirs in these wells consist of multiple stacked sandstones. Various factors were examined, including BHA design, drilling parameters, bit types, formation lithology and well azimuthal orientation. A variety of BHA configurations were tested but none of these changes had an appreciable effect on vibration levels. Although the drilling parameters were kept relatively constant, WOB and RPM were adjusted periodically in response to high surface torque (which correlates with stick-slip vibration). It became apparent that a higher degree vibration resulted from more aggressive PDC bits.
Two Drillstring Dynamics Sensor (DDS™) subs were run during drilling of the 6-1/8-in. production sections in several wells, because a single sensor placed near the bit in previous wells was frequently damaged. The lower sub, placed approximately 20 ft from the bit, was made up to the MWD tool so that data could be transmitted in real-time data while drilling. The upper DDS sub was placed near the top of the heavy-weight drillpipe (HWDP), approximately 800 ft from the bit, in a standalone, record-only configuration. Analysis of data from these widely separated sensors clearly showed that torsional vibration is transmitted along the drillstring with very little attenuation. An important effect of torsional vibration is lateral vibration, which can be initiated during the slip phase of stick-slip as the bit and BHA spin freely at several times the surface RPM. The data analysis suggests that lateral shocks associated with stick-slip are most likely responsible for the downhole tool and component failures. Also, high torque in the drillstring during torsional vibration probably contributed to drillpipe and drillstring fatigue and the belled boxes found in both HWDP and drill collars. The results of the study led to recommendations to the operator that have been effective in reducing harmful vibrations and in improving the performance of BHA components.
Campos Basin, offshore Brazil
The narrow margin between pore pressure and fracture gradient makes drilling in the Campos Basin very challenging. The drilling problems and NPT experienced by Petrobras in the Campos Basin are either hydraulics-related issues (80%), such as lost circulation, kicks, stuck pipe, poor hole cleaning, or vibration-related issues (20%), such as MWD and BHA failure, low ROP, and excessive bit wear. These issues usually lead to time-consuming and expensive remedial operations. Drilling optimization operations have significantly improved drilling efficiency, reduced drilling costs, and reduced NPT. Halliburton’s Sperry Drilling services developed a special, cost-effective optimization program that emphasized the use of real-time pore pressure evaluation, pressure-while-drilling (PWD), and vibration sensors. This was in effect, a “slimmed down” version of the ADT service that focused on hydraulics management and drillstring integrity.
At the beginning of the campaign, the accuracy of the pore-pressure prediction software was confirmed by pressure data from a wireline formation tester.
PWD is particularly valuable in this basin for its range of applications, from monitoring ECD and hole cleaning to preventing stuck pipe and lost circulation. The downhole PWD data proved to be more accurate and reliable in providing early warning of drilling problems, such as cuttings buildup (packoff) and differential sticking, than surface data. Based on PWD data, the ADT service engineer recognized cuttings buildup in a well and recommended a new drilling procedure to ensure borehole cleanup before stopping the mud pumps following slide drilling. This procedure was adopted by the operator and drillers and has proved valuable in preventing costly hole-packoff incidents. Since the drilling team began working closely with the ADT service engineer to monitor the PWD data, there have not been any new stuck-pipe incidents. Changes in PWD differential pressure were also used to monitor the variations in weight-on-bit, which, in turn, was used to evaluate and optimize motor performance and increase the rate of penetration. This capability has proved especially important in high-angle wells.
After an incident in which unchecked BHA whirl and medium to high levels of lateral shock, undetected at the surface, resulted in failure of the PWD tool and severe borehole washout, the DDS sensor has been run routinely in almost every well to monitor vibration. The WHIRL™ program, finite-element-based BHA dynamics modeling software, was used avoid harmful resonance by predicting critical rotary speeds before each run. The number of vibration-related tool failures has been significantly reduced since implementing the vibration services.
The campaign to optimize drilling efficiency and reduce NPT in the Campos Basin demonstrated that using fit-for-purpose tools, such as real-time PWD and vibration monitoring, together with knowledgeable personnel experienced in the local drilling issues, a successful program could be carried out at little additional cost.