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Identifying and expanding the potential in exploratory wells

By  KJ Tan & Cole Yue Tuesday, 22 February 2011 22:19
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Solid expandable tubulars (SET) help facilitate discovery and assessment, as Enventure Asia Pacific's general manager KJ Tan and operations manager Cole Yue explain.

Although clastic sedimentary deposits, such as those prevalent in seas between the Pacific and Indian oceans, make for prolific reservoir plays, the geology of these formations also results in dynamic drilling conditions. The tectonics in Asia Pacific have contributed to inconsistent strata that compound the usual challenges of downhole operations. Undulating formations and shifting plates test the agility of well designs to adapt and address the unexpected. Exploratory wells are particularly challenging with the unpredictable to be expected. Considering multiple 'what if ' scenarios enables the incorporation of new technologies as proactive strategies and mitigating contingencies to better address the challenges of exploration.

The proactive strategy of logging-while-drilling (LWD) with a pressure-while-drilling (PWD) precision can accurately predict lithology, pore pressure, and dynamic downhole conditions. This information enables real-time decisions on key variables, such as equivalent circulating densities (ECD), where there is little room for error. A circumscribed frac gradient/pore pressure margin, loss of circulation, and depleted formations often require more than a change in mud weight to rectify the situation and continue drilling operations.

Another frequently employed strategy uses solid expandable technology, which provides the means to mitigate trouble zones without sacrificing hole size. When deep reserves are in play, lengthy drillstring issues also impact wellbore construction and mandate that adequate hole size be maintained. While managing hole size to reach extreme depths has presented challenges, SET makes drilling exploratory wells more feasible. As with other enabling technologies, the recent emphasis on expandable applications has focused on opportunity rather than need, resulting in reduced non-productive time , streamlined operations, and greater project feasibility.

set openholeOffshore Asia an Enventure 113/4 x 133/8in 68lb/ft SET OHL extended casing depth with minimal loss of hole size.

Productive formations in the Asia Pacific region are found in deltaic, fluvial environments. These formations range from shallow-depth reservoirs to heavily faulted deep basins. In addition to the prevalent clastic sediment, stacked pay sections include conglomerates, igneous rock, and carbonates. Ongoing tectonic movement, earthquakes, underwater landslides, lost circulation zones, and mudflows increase geological complexity.

The upheaval found in sandstone, shale and reactive clays make for troublesome depleted formations and pressure regression formations, narrow fracture gradient ratios, and ECD issues. The customary response has been to either set casing or perform multiple cement squeezes. Both solutions are reactive and offer limited value to the drilling program. These approaches more often complicate the already technical challenge of exploration drilling.

Staying on track

The Malaysian offshore has estimated reserves of 5.5 billion barrels of oil and over 70tcf of natural gas, according to the 2010 BP Statistical Review of World Energy. These estimates and production of over 700,000bo/d prompted a major operator to drill several exploratory wells off the coast of Sarawak, east Malaysia. While drilling the initial well from a drillship in 5000ft of water, an unstable section was penetrated at approximately 7500ft. Wellbore instability occurred while drilling a 141/4in straight hole with synthetic-based mud in a shale formation. Wellbore trajectory was vital for optimum evaluation. The most reliable option to overcome the trouble zone was to set casing, which would cause an unsuitable change to wellbore architecture.

An extra casing string would preserve trajectory but cost a casing point and jeopardize adequate hole size at total depth. Subsequent well evaluation phases depended on maximizing inside diameter to ensure proper deployment of the necessary investigative tools and technology. The operator decided against sacrificing hole size and installed an Enventure expandable open hole liner (OHL) to stabilize the problem shale section and stay on track.

An expandable OHL, over 815ft (preexpanded) in length, was run with the shoe set at approximately 8200ft. The 113/4 x 133/8in expandable system covered 662ft of open hole and tied back into 133/8in 72# casing. Following expansion, a 30-minute 3000psi leak off test verified OHL integrity.

The expandable solution dealt with the problem so effectively that the operator planned a smaller OHL system as a contingency option.

Extenuating circumstances with the drillship later shut down operations and required temporary plugging of the well for a few months. When drilling resumed, the operator needed the planned expandable contingency, a 75/8 x 95/8in OHL, to keep the well on its original course.

Traditional approaches to addressing these types of exploratory situations too often strain the economic viability and technical capability of the drilling plan. A restricted ID necessitates more costly and less robust slimhole equipment for evaluation, a time consuming and riskier course of action preferably avoided.

OHLEnventure expandable OHL allows continued drilling with optimal hole diameter.

Another operator drilling in the Timor Sea faced a similar decision – how best to stabilize the wellbore without compromising hole size in an exploratory well. Trips into formations with minimal offset data or comparable logs often result in a deviation from the original design, as was the case with this project in the Browse Basin. Located on Australia's northwestern continental shelf, the basin consists of gas/condensate fields located centrally and on its outer perimeters. With reservoir rock found in the western part of the basin, an estimate of oil and gas production potential required further exploration.

This operator ran into trouble while drilling a carbonate and sand/shale section at over 12,400ft in approximately 1600ft of water. Mitigating this zone required setting the 95/8in casing string higher in the hole than planned. After drilling out the casing shoe, a fracture gradient was encountered lower than expected. The mud weight required to drill the remaining interval was less than planned for the more competent shale section below. Adding another casing string to isolate the depleted zone would jeopardize the ability to reach TD with the planned hole size. Cement squeezes to regain formation integrity were unsuccessful, and the alternatives of sidetracking or pulling and re-running the 95/8in string were time-consuming and expensive.

A 'less than optimal' shoe test or formation integrity test (FIT) prevented reaching the desired TD. To get back on track, the operator drilled approximately 300ft out of the depleted zone and into a solid formation and ran Enventure's 75/8 x 95/8in expandable OHL, which extended the shoe of the 75/8in casing and provided the desired FIT needed to drill the remainder of the well and keep the original production string size. This expandable liner installation required rapid mobilization of equipment and personnel from outside the country to meet a scheduled timeline, diminishing wellbore instability while conserving hole size to maintain the original well design and avoiding costly sidetracking or re-running of casing.

Reactive clays and sandstone present problems throughout the Asia Pacific. An exploration well in the Gulf of Tonkin encountered a problem at around 3280ft in over 325ft of water. The downhole condition of the clay formation required setting casing higher than originally planned. Deviating from the wellbore design put the TD hole size at risk. Too small a hole would limit the size of logging tools, forcing the operator to use the more expensive and less reliable slimhole tools.

system setOffshore Australia, an Enventure 75/8 x 95/8in SET OHL installed over a 421ft interval isolated a weak section while conserving hole size.

Rather than compromise original project design, the Enventure 113/4 x 133/8in OHL was employed to extend the shoe of the original 133/8in casing. The solid expandable liner saved a casing point and kept the wellbore design on track. Reaching TD with the optimal hole size saved the well and enabled production logging and the designed completion.

Technical enhancements have contributed to solid expandable systems being able to be deployed deeper, longer, and in more extreme conditions. The application realm for these systems encompasses use as a diagnostic tool, multiple expandable systems in a single wellbore, and extended-reach drilling and completions. Although initially conceived as a technology to help resolve unexpected downhole challenges, expandable tubulars realize greater return and attain increased reliability when incorporated into the plan whether for exploratory, development, repair and workover, or brownfield projects. These systems have also been used as part of a larger enabling package, coupled with other technologies such as swellable elastomers, pinpoint fracturing, jet perforating, and smart completions.

Whether tripping into the unknown for assessment purposes or leveraging extensive offset data to optimize recovery, wellbore designs are subject to change. incorporating SET systems from the outset facilitates a slimmer well profile while still maximizing hole size at TD, and also reduces drilling time to TD as well as overall drilling costs. Operators with offshore projects have used these systems to capitalize on the existing infrastructure with sidetracking or slot recovery operations. Reservoirs that were previously out of reach because of economic factors or technical constraints have become much more accessible.

As planned-in contingencies, SET allow for aggressive well designs in the event of trouble zones or unexpected problems. Exploratory well designs have used expandable systems as contingency strings to achieve set targets and discovery objectives.

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