Sidetracking a preexisting drilled and cased wellbore poses numerous challenges. When sidetracking in an openhole environment, additional verifications of hole conditions are needed, which contribute additional unique challenges. In this type of wellbore, the operators must plan by selecting the sidetracking depth and then ensure that all the objectives are met from a well authorization for expenditure standpoint and geological target perspective. The quality of the openhole window or rathole is of immense concern to operators because this quality ensures that the bottomhole assemblies (BHAs) will pass through the rathole without difficulty. The openhole gauge must be confirmed because it can pose additional risks that might lead to costly multiple trips into the well to ensure that the rathole is in good condition. This paper presents a unique case study in which the operator, Kuwait Oil Company (KOC), was faced with the possibility of geological losses at the kickoff in the wellbore while attempting to sidetrack an existing wellbore. The operator contacted an oilfield services company and requested a unique technical solution to precisely sidetrack the wellbore in the difficult formation containing fractured dolomites, which are known to cause severe to complete losses. While drilling the 12.25-in. section, the BHA became stuck in the fractured dolomitic limestone formation. In this section, the operator had previously experienced severe to complete losses. Because fishing attempts to free the stuck BHA were not successful, the operator decided to sidetrack the wellbore in the open hole using an openhole whipstock. A casedhole sidetrack option was ruled out because reactive swelling shales with producing sands were located above the sidetrack depth; therefore, combining these zones was not practical because of the low-mud-weight limit required for drilling the fractured dolomite below the target depth. In addition, because only 70 ft of open hole existed between the 13.375-in. casing shoe and the stuck BHA in the hole, sidetracking with a cement plug was nearly impossible. The openhole wellbore was logged with a caliper to confirm the wellbore gauge. Prejob planning consisted of understanding the compressive rock strength from the offset wells to identify the lithological challenges unique to this application. A hazard analysis risk-control method was adopted to identify the risks and apply appropriate mitigation measures. An operating parameters plan was formulated by the engineering team and discussed with the operator and service company personnel and followed throughout the job. The wellbore was successfully sidetracked in the 12.25-in. section in a single run using an openhole whipstock, avoiding the loss zone, and resulting in additional cost savings to the operator. The condition of the sidetracked rathole enabled smooth passage of the directional BHA to meet the directional objectives. Furthermore, the openhole whipstock operation eliminated the need for multiple cement plugs in the sidetrack (in view of severe loss zones below) as well as the time required for drilling with a dedicated motor BHA for openhole sidetracking operation, saving the operator a minimum of 6 days of rig time. This operation was the first successful 12.25-in. openhole sidetrack operation in the Middle East, Asia-Pacific, and sub-Sahara Africa regions. As a result of this successful operation, the operator is proactively recommending the new solution across the entire KOC organization for wells with similar scenarios. By applying this unique and reliable openhole whipstock technical solution, the drilling team was able to deliver a successful well based on the original casing plan without any need for further sidetracks or changes to the wellbore casing design.
Kuwait Oil Company (KOC) launched North Kuwait Gas Development Project in 2007 to fast track production of gas/light oil from deep HPHT Jurassic reservoirs in six fields. The exploratory wells require a large-hole casing design to isolate problematic formations and allow the operator to reach the deep HPHT target zones with maximum hole size. One of the most difficult/problematic hole sections is Zubair to Hith, traditionally drilled with 16-in bits. The formations are interbedded and consist of abrasive/pyritic sandstone, reactive shale, limestone and anhydrites with UCS that varies between 5-30kpsi. Historically, to complete the 3900ft section required more than three PDC bits or a combination of 4-5 PDC/TCI. To reduce costs, the drilling team developed the first slim hole well plan and casing string design to explore Cretaceous formations in Northern Kuwait. The objective would be to downsize the hole section from Zubair to Hith to 9¼-in and drill to section TD with three PDC bits. The production casing would then be run from surface to TD instead of the conventional liner tie-back and cementing process. The section was completed using the latest motor/PDC technologies in two bit runs saving 55% drilling time and the expense of a third bit. The operation set a new Kuwaiti record for the single longest 7¾-in production casing string (13,625ft) and cementing back to surface. Cement log indicated a good bond over all three zones of interest. The downsizing strategy and change in casing string design saved the operator over $1 million USD compared to a large hole and liner-string design. Production testing in Zubair, Ratawi and Makhul was positive, indicating a significant new oil discovery. The entire operation was completed with zero HSE related complications.
A 16-in. section is one of the most challenging sections to drill in southeast Kuwait. The section consists primarily of highly interbedded carbonates, anhydrites, shale, and marl intervals, with significant variations in compressive strength that range from 3 to 25 kpsi. There are potential loss of circulation zones in the Tayarat and Dammam formations, which complicates the drilling performance, required flow rate, and other drilling parameters to be controlled. Bit balling of the cutting structure was expected in some intervals where water-based mud was typically used. The drilling assembly was a pendulum rotary design for vertical wells. A positive displacement motor (PDM) assembly is typically used in directional wells. In addition to the loss potential and long section interval, insufficient hydraulics and the high weight on bit (WOB) necessary in the Dammam to Ahmadi carbonates led to the premature failure of the cutting structure and bearings. A durable and reliable bit design was needed to drill the entire interval and the subsequent well to help reduce overall drilling costs. Collaboration between the operator’s drilling team and the bit supplier team resulted in successfully drilling a challenging 16-in. section and multiple additional wells. Rock types, performance, and compressive strength were analyzed using an advanced software program to determine the required design changes to develop the new tungsten carbide insert (TCI) bit. The goal of the new design was to improve hydraulics configurations, create durable and robust rubber seals, and design new advanced cutting structure materials. The new bit design provided exceptional performance in multiple wells in the 16-in. section in southeast Kuwait. The bit was used to drill three wells, with a cumulative footage of more than 9,000 ft in more than 240 drilling hours. Approximately two million revolutions were achieved with effective bearings and slight normal wear to the bit cutting structure elements. The new bit design helped to drill longer intervals and multiple wells with no risk of premature failure; it also helped to reduce drilling time and bit costs. A further test was implemented in directional drilling wells where the inclination was up to 50°.
Almost all worldwide oilfields which have been producing for a long time have reached brown level of maturity. Given the foreseeable increase in global oil demand and consumption in the near future, operators need to be innovative in finding new resources and developing plans to meet the production demand. Oilfield service companies have parallely kept the pace by continuously improving their technologies and evolving their product offering to support activities from exploration to production phases. Sabriyah Lower Burgan reservoir in North Kuwait has been producing for a long time after its discovery. Since oil production is mostly from massive and coal sand members for long time, exploitation of new reservoir is needed to sustain the field productivity. The non-traditional siderite layer (SID3) had been identified by Kuwait Oil Company (KOC) as a potential source of incremental oil production. The reservoir has a lower productivity index and cannot flow significantly in vertical wells. SID3 has a relatively thin layer (5-20 ft TVD) with high lateral variations and presence of regional coal, making it highly challenging for drilling operations. There were many cases of wellbore instability while drilling wells in Lower Burgan in the past. Geological uncertainty generated by pinch out sand bodies in the reservoir section, led to scratching unstable shales and coal layers prone to collapse, thus resulting in stuck pipe events due to hole pack-off. Large volume and size of cavings, restricted circulation, high over pull, tight spots while tripping, and severe time-dependent wellbore instability due to shale sloughing contributed to the event and resulted in costly sidetracks. The geomechanics study is a key technology in the oilfield industry to prevent wellbore instability, via optimization of well trajectory and proper mud weight selection. Application of the study outputs helps minimize drilling risk, diminish non-productive time and ensure well completion within the expected deadline. Horizontal well design is deemed as the most effective way to exploit the SID3 layer, and the application of High Definition Distance-To-Boundary (DTB HD) technology can help the operator mitigate geological uncertainties, maximize reservoir contact and hence boost production rate. Integration of proactive Distance-To-Boundary technology, geomechanics study and meticulous pre-drill preparation have led to the successful completion of several horizontal wells in the SID3 layer. As a result, the operator gained a significant increase in oil production compared to the vertical wells. The well production also benefited from advanced completions technology with inflow control devices, handling other production related matters like presence of shale/silty layers for optimum hydrocarbon recovery. This proves that high-end technologies, coupled with multi discipline domain expertise and high-performance teamwork, is key to sustain the industry's ability to respond to the increase in global energy demand.
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