A workflow that combines optimization of the drill string and bottomhole assembly (BHA) design during well planning and then applies advanced surveillance tools to a well-trained drilling crew yields reduced vibrations, higher drilling rates, and less trouble cost. This methodology is based on the premise that an efficient drilling operation requires optimized tool designs, advanced diagnostics using real-time drilling parameters, and onsite training of efficient drilling practices and the proper use of rig control systems. The use of efficient modeling procedures to compare alternative drill string and BHA designs provides valuable insights into the string and tool selection process. A method to select the optimal stabilizer contact locations for the BHA tools helps to avoid lateral vibration dysfunctions, and a torsional vibration model can quickly evaluate the resistance of alternative string designs to harmful torsional stick-slip vibrations. Provided the proper hardware, a well-trained driller can be more effective with automated drilling performance evaluation tools that provide real-time drilling parameter recommendations based on optimizing Mechanical Specific Energy (MSE), torsional vibration stick-slip severity, and Rate of Penetration (ROP). BHA lateral vibrations modeling is field-proven and has been applied globally. One case study will show an application of the model to select a BHA design with specified rotary speed sweet spot. The torsional vibration model can be used in both a design process and in a real-time surveillance mode. In one case study, stick-slip vibrations were too severe to drill ahead with a tapered string design that was selected to lower the equivalent circulating density (ECD). The model helped identify the increase in stick-slip resistance obtained by substituting a portion of the smaller pipe with larger pipe. A real-time surveillance tool provides automated drilling performance analysis and makes recommendations to the driller on bit weight and rotary speed. The recommendations are based on the torsional vibration model results, operating in a surveillance mode, and the MSE and ROP. Rig control systems impact drilling dynamics and efficiency in ways that are not well understood by most drillers, and training on awareness and mitigation of these effects can avoid severe dysfunctions.
Accurate wellbore geometric placement is fundamental to achieve the objective of maximizing hydrocarbon production and recovery. It is especially essential in real time to drill complex 3D well trajectories that penetrate multiple thin geological targets. Accurate placement is also critical to avoid catastrophic subsurface collision of nearby offset wells. This is critical in particular in this gigantic field with over 800 wells drilled by two different operators with several hundreds more wells to follow. Almost all Bottom Hole Assemblies (BHA) run in this field include a Measurement-While-Drilling (MWD) survey tool to survey the wellbore while drilling. The MWD is a magnetic survey tool that is subject to errors that limit the magnetic survey accuracy. One of the main sources of error is the variation in the local magnetic field due to the crustal anomalies in this field. The magnetic surveys can provide an accurate geometric well placement by incorporating the knowledge of the local magnetic field disturbance to the main geomagnetic field model and by compensating for the drill-string magnetic interference. The Geomagnetic Referencing Service (GRS) technique based on magnetic surveys was introduced in this field. This technique utilizes the local magnetic data that is measured over the field by acquiring a high-definition airborne gravity and magnetic survey. The accuracy of the geometric well trajectory of the first oil producer well is compared between the MWD, cased gyroscopic and GRS surveys and the advantage of the GRS is presented. The benefits of applying GRS in real-time while drilling is paramount, where it provides an accurate well position in real time when corrections to the well trajectory are still possible. It prevents the costly sidetracks if the post drilling gyroscopic survey shows the well has missed its target. In most cases, GRS is an alternative to the gyroscopic surveys where it provides magnetic survey accuracy that is comparable to the casing gyroscopic survey tools. Hence, it saves the cost and risk of running gyroscopic survey tool as well as the cost of the extra rig time required to run a gyroscopic tool after drilling.
The island development strategy of the giant offshore oilfield requires the use of extended reach drilling (ERD) design wells. Compared to the typical wells drilled from the wellhead towers in the same field, higher inclinations are required in both the surface hole and intermediate hole to facilitate drilling three dimensional wells of more than 35, 000 ft. While the challenges of drilling the intermediate hole at higher angles had been identified early on due to field experience, the challenges leading to stuck pipe events encountered in the surface hole were not anticipated due to limited experience drilling high angle surface holes in the region. Historically total loss of returns has been a common issue in the region when drilling the surface hole. Typically when drilling from the Jack Ups, the wells are drilled with sea water and high viscosity sweeps once total losses has been encountered. Any potential aquifer flows are diverted overboard. In order to divert the aquifer flows on the newly built Artificial Islands, the fluids must be pumped 200 or more meters to the gulf. Mud cap drilling (drilling with seawater down the drill string with heavy mud in the annulus to control well flows) was implemented to solve the issue of losses and flows on the island. The early wells with surface holes drilled at high angle experienced stuck pipe while tripping out of the hole after reaching casing point, leading to significant non-productive time (NPT) and risking project objectives and planned designs. A detailed investigation was performed, including running six arm caliper logs to better understand the mechanism for stuck pipe events. After analyzing and understanding the issue, operational practices and bottom hole assembly designs have been changed to reduce the stuck pipe risk, and specially designed stabilizers have been manufactured and used to mitigate stuck pipe events. Geologically, significant data gathering within the overburden sequence to characterize lithological, stratigraphic, and diagenetic heterogeneities, as well as structural discontinuities, has improved understanding of aspect ratio and vertical scale of features being drilled that may have caused the previous hole morphology effects. No stuck pipe events have been experienced to date in the surface hole due to the same effects after implementation of the new equipment designs and improved drilling practices.
Pressure core is the gold standard of reservoir saturation determination retaining gas and oil in an enclosed container preventing hydrocarbon loss to the mud system during retrieval of the core to surface. The technique is underutilized in the oil industry due to safety concerns, short coring lengths (3 to 7 feet) per trip and small core diameters (1.6 to 2.6 inches). A new elevated pressure coring system addresses these concerns. The system does not maintain true formation pressure but reduces the core barrel pressure to less than 1,000 psia for safer working conditions on surface. Upon arrival at surface, the core barrel and accompanying gas/liquid collection canisters are blown down and gas/liquid volumes measured and analysed. Free oil is gathered and stored for later analyses. Core diameter is 4 inches and core length is 10 feet. Zakum Development Company (ZADCO) operates a gas injection pilot in a large carbonate oil reservoir. There was a need to determine field remaining oil saturation after gas flood (ROSg) in a gas injection pilot. Pressure coring was selected as the best technology to obtain this data. This paper covers the planning and implementation of a successful elevated pressure coring operation in the U.A.E., the operational aspects, special core handling techniques, issues encountered and solved. Recommendations are made for future pressure coring operations. A follow up paper will cover the core and fluid analyses aspects.
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