Coning
Authors: Richardson J.G. , Sangree J.B. , Sneider R.M. 
Published: December 2007

Technology Today Series articles provide useful summary informationon both classic and emerging concepts in petroleum engineering. Purpose: To provide the general reader with a basic understanding of a significantconcept, technique, or development within a specific area of technology.

One of the decisions that must be made in the development of production froma field is in placement of the completion intervals in the wells. placement ofthe completion intervals in the wells. Studies of the seismic, core, log, andwell test data are required to identify the potentially separate zones in thereservoir. Each zone that is separated from others by impermeable barriers, which potentially extend over large areas of the reservoir, should be opened toproduction (and injection if needed) to ensure drainage of oil from the zone. When geologic studies of the data indicate that the depositional environmentwould favor barriers of limited areal extent, calculations can be made toevaluate the potential coning of water or gas to aid in the location potentialconing of water or gas to aid in the location of the completion intervals, Forexample, completion in the interval above a barrier could limit coming, ofwater. Oil beneath the barrier will drain around the barrier by gravitysegregation and be recovered if the width is not too large. Both the coning andgravity drainage problems can be assessed with the mathematical modelsdiscussed. The model for evaluating coning behavior in the presence of abarrier of limited radial extent around a presence of a barrier of limitedradial extent around a well is shown in Fig. 1. Radial barriers can limitconing because the pressure drawdown caused by production is less at the edgeof the barrier than that production is less at the edge of the barrier thanthat at the well in the absence of the barrier. Thus the rise of the water/oilcontact against the force of gravity will be less between the drainage radiusof the well and the edge of the barrier. Karp et al proposed a model involvingcreation of horizontal proposed a model involving creation of horizontalbarriers to control coning.

An example application of Eq. 1 is shown in Fig. 2 to illustrate the effectof barrier radius on critical oil production rates without production of gas. In this production rates without production of gas. In this example case, thecritical production rate would be about 5,000 STB/D [800 stock-tank, in m3/d]in the absence of barriers and more than 12,000 STB/D [1900 stock-tank m3/d] ifthe well were perforated in the bottom 100 ft [30 mi of a 400-ft [120-m] oilcolumn below a barrier 100 ft [30 m] in radius. In practice, coning is usuallycontrolled by working wells over to reperforate above a shale barrier to avoidwater coning or below a barrier to avoid gas coning. Another method involvingnew drilling technology is to drill horizontal wellbores. Horizontal wells canincrease critical coning rates three- to fivefold above those for verticalwells. Fine-grid computer models based on detailed geologic descriptions of thesize and distribution of shale barriers can be used to evaluate variouscompletion strategies. Simulators also can be used to improve reservoirdescription by matching production histories of wells once gas or water coninghas occurred.

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