Introduction

The search for new sources of oil, and particularly gas, is leading the industry to drill particularly gas, is leading the industry to drill ever deeper wells. A depth of 15,000 ft was first passed in 1938, 20,000 ft was reached in 1939, passed in 1938, 20,000 ft was reached in 1939, followed by 25,000 ft in 1958, and 30,000 ft in 1972. The current US record depth is 31,441 ft. As the total depth increases, not only does the rock to be drilled become stronger, but increasing pressure and temperature induce plasticity and pressure and temperature induce plasticity and "chip hold-down" effects that make it more difficult to remove cuttings from the workfront. In addition to the reduction in rate of the drilling process itself, other activities become more complex and time-consuming, for example, tripping, running and cementing casing, and logging and coring activities.

We have analyzed the different tasks involved in drilling deep wells, in order to identify those activities that contribute most to the overall cost. These are therefore expected to be the activities where future efforts in research and development should provide the greatest reductions in total cost.

2. OVERALL EFFECTS OF INCREASING DEPTH

We have obtained information from a large drilling database that contains data on over 3000wells, drilled in all parts of the world, both on land and offshore, during the period 1977–1988.The majority of these wells were for exploration rather than production. Some of the major findings include the following:

For 3111 wells analyzed, of all depths, on land and offshore, the total rig time included 12.1%spent in well completion and 9.9% in moving between sites. (Figure 1). If the data are separated into wells drilled on land and offshore, it is found that the major difference is in the time spent incompletion (11.4% onshore vs 15.6% offshore, and in moving (11.2% onshore vs 5.4% offshore). The fraction of time spent in drilling is not very different in the two cases (26.8% land vs 22.3%offshore). The reason for these differences is first that the completion of a subsea well is more time-consuming than the equivalent job on land, and second, proportionately more time is spent in moving a land rig because the rig has to be brokendown into easily-transportable units and then reassembled at the new site. Offshore, the whole rig moves as one unit. Further, the average length of time taken to drill a land well is typically less than for an offshore project, and so the moving time becomes a greater fraction of the whole.

These data may be compared with information compiled by the UK Department of Energy concerning offshore development drilling activities (1)Here, it was found that completions occupy 12% of the time, a slightly lower value than for the offshore exploration wells. This is perhaps because in development drilling the completion procedures are more standardized. Development drilling, on the other hand, occupies more time - 26% vs 22.3%.This may be because development wells offshore are frequently deviated, and therefore slower to drill, in spite of the fact that in development drilling, the necessary operating parameters are better known, and the job should be more straightforward(Note that as a proof of the latter point, more than twice as much time was spent in circulating and conditioning the exploration wells (4.8%) as compared with the development wells (2%).

For all this data, the most important observation is, however, that the largest single sector is the drilling time, followed by the triptime. As the time spent in moving and completion is not strictly part of the drilling process, our subsequent analysis will not include these times.

We now examine the way that the different time sectors change as a function of well depth. Removing the completion and moving times from all the data, we find that drilling now occupies 33.7%, and tripping accounts for 16.7%. Running casing accounts for the third largest sector, at 12%.

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