Slim-hole continuous-core drilling and on-site near-real-time core analysisoffer a new dimension for oil and gas exploration, especially in remote, difficult, and environmentally sensitive areas. Technologies borrowed from themining and oilfield drilling industries coupled with newly developedtechnologies were used to create a new drilling and evaluation system designedaround continuous coring, slim holes, and inverse loggingi.e., the logging ofthe actual rock as it is retrieved. This new system could cause a paradigmshift in exploration.
A slim-hole well is one in which 90% or more of the length of the well isdrilled with bits less than 7 in. [less than 17.8 cm] in diameter. There aretwo basic reasons for drilling a slim hole. The first is very simple and basic:better economics. The second is borrowed from the continuous-core-miningdrilling industry: to achieve a high-percentage core recovery. Slim-holetechnology is not new. Both the explorationist and the exploitationistrecognize the possibility of using a small-diameter wellbore to reduce overalldrilling costs. In the 1950's, a major operator launched an initiative to drillslim-hole exploitation wells in Utah, Louisiana, Mississippi, Arkansas, andOklahoma, and concluded that slim-hole wells could be cost effective. However, interest in slim-hole exploitation drilling waned in the 1960's and did notre-emerge until the 1970's. A slim-hole drilling system was developed in Swedento explore and exploit some of the small, shallow reservoirs there. Dah cited a75% cost savings in this approach over conventional drilling practices. Thesuccess in Sweden encouraged practices. The success in Sweden encouragedslim-hole drilling in the U.K. and in the Paris basin in France. Costeffectiveness Paris basin in France. Cost effectiveness was again cited. Explorationists also recognized that slim-hole technology would be useful fordrilling in such remote areas as Indonesia, particularly Irian Jaya. The smallsize of the overall drilling system and the reduction in wellbore size greatlyaffected the logistical cost for drilling these wells. The greatest use ofslim-hole technology is for continuous coring in the mining industry. The onlysure way to evaluate whether an ore body is large and has a mineral gradesufficient to justify the cost of sinking shafts or embarking on a costly opencut is to obtain core to delineate the ore body. South Africa, the leader incontinuous-coring mining drilling, has developed deep gold deposits there, withsome mines below 10,000 ft [3048 m]. Continuous-coring mining drilling is sovast that it is supported by a subindustry that is totally unrelated to theoilfield drilling industry. Special manufacturers design and build drillingmachines, drill bits, tubulars, core barrels, and other required tools. Infact, the slim-hole mining drilling industry has its own association, the Intl. Drilling Federation (IDF), complete with a quarterly publication called DrillBits. Continuous-coring mining drilling operations are currently booming in theU.S. (gold and coal), Canada (mainly nickel), and Australia (various minerals). Over the years, an ongoing interest in continuously coring oil and gas wellshas existed. During the late 1950's and early 1960's, a continuous-coringsystem was developed and used to drill a number of shallow wells for oil andgas. Unique to this continuous-coring system was the design to reversecirculate the core from the well while drilling ahead. During the early 1970's, a well was continuously cored to 11,600 ft [3536 m] with the slim-hole coringsystem. This record still stands as the deepest continuously cored well in anoil and gas sedimentary environment. According to the Oil and Gas J., a 50 %cost improvement was achieved over a conventional well drilled to 6,000 ft[1829 m], and explorationists realized the advantages of continuous coring andthe slim-hole system, especially for drilling in remote areas. The concept ofcontinuous coring as an alternative exploration strategy was championed in theearly 1970's in Australia, where a series of oil and gas exploration wells weredrilled in the Canning basin. During the mid-1980's, two wells werecontinuously cored in the Permian Age Delaware basin in Texas and New Mexico. Both wells recovered more than 11,000 ft [3353 m] of continuous core. During1987–89, Amoco Production Co. drilled more than 40,000 ft Production Co.drilled more than 40,000 ft [12 200 m] of continuous core in Oklahoma, Michigan, Kansas, Colorado, and Texas. Some of that drilling is discussed inthis paper. With the apparent interest in slim-hole and continuous-coredrilling during the past 40 years, why haven't both approaches become morepopular in the oil and gas exploration and exploitation sectors? Is there aplace for slim-hole and continuous-core drilling for oil and gas explorationand exploitation?