Integrated Optimization on a Long Horizontal Well Length
- Hyun Cho (Kellogg Brown & Root Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- April 2003
- Document Type
- Journal Paper
- 81 - 88
- 2003. Society of Petroleum Engineers
- 2 Well Completion, 1.8 Formation Damage, 5.6.8 Well Performance Monitoring, Inflow Performance, 5.7 Reserves Evaluation, 1.6 Drilling Operations, 5.7.2 Recovery Factors, 1.6.6 Directional Drilling, 2.3.4 Real-time Optimization, 5.7.5 Economic Evaluations, 4.6.2 Liquified Natural Gas (LNG), 4.6 Natural Gas
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This paper presents a methodology developed to estimate the optimum horizontal well length based on total economics and on the productivity index (PI) change associated with frictional loss effects in the long section of the horizontal wellbore. An integrated approach is proposed for quantitative analysis of the parameters affecting productivity and economics to achieve the overall optimum performance of a horizontal well project. This study shows that production is no longer proportional to the horizontal well length after a critical point. The critical point is calculated as a function of the length of the horizontal production section, flow rate, drawdown pressure, and other parameters. A step-by-step calculation procedure with examples is described with sets of graphical charts developed for illustrating the relationships between the various parameters.
As the length of a horizontal well increases, its drainage area also increases. Initially, it was believed that a horizontal well should be as long as possible. Current drilling technology allows wells to be drilled several thousands of feet long. However, a factor exists that can possibly limit the useful length of a horizontal well (i.e., frictional losses in the wellbore).1 Long horizontal wells or high flow rates result in an increase in frictional losses. This may be comparable with the drawdown at the producing end of the well. A portion of the downhole well would then be unproductive because of frictional losses. Recent experience with horizontal wells has revealed that in many circumstances, the inflow performance of horizontal wells does not match with the expected productivity, and their deliverability may be affected by frictional losses along the wellbore.2 This effect has serious implications where the horizontal well section is very long because the PI is no longer directly proportional to the well length.
A survey3 shows that horizontal drilling can be used on almost any reservoir setting, and its success rate reaches up to 95%. It provides convincing evidence that the implementation of horizontal drilling techniques has become almost routine. However, the economic success of horizontal projects has not been widespread because only 54% were reported as economically successful.3 Another point in horizontal drilling is the incremental expense over conventional drilling. It costs approximately 1.4 to 3 times4-7 more to drill a horizontal well than a vertical well, depending on the drilling method and the completion technique used.
Another horizontal well survey6 reports that 62 out of 91 (68.1%) wells were drilled up to full reservoir length, and 70 out of 91 (76.9%) wells were drilled to more than 90% of the reservoir length. Hence, the question is, can a horizontal well length over 90% of the reservoir be as economical in view of the optimum well construction? When the general economic success rates are considered, it is difficult to conclude that wells penetrating over 90% of the reservoir formation are well optimized. Optimum well length for the horizontal well is a function of revenue increase, hydrocarbon price, drainage area, friction loss, PI, early-time production increase, well-construction costs, and reserve increase. Therefore, it is important to consider optimum horizontal well construction.
The purpose of this paper is to present the effects of long horizontal wells on a production rate associated with the effects of friction pressure losses in the wellbore. The methodology to determine the economical horizontal well length as one of the parts of the optimum horizontal well construction is also proposed.
Horizontal Wellbore Pressure Profile
Giger8 and Joshi9 presented a pressure profile created by 3D steady-state flow to a horizontal well located inside an ellipsoidal drainage. Once the pressure distribution is known, oil-production rates can be calculated with Darcy's law. The pressure distribution caused by steady-state flow to the horizontal well is approximated by subdividing the 3D flow problem into two 2D flow, as Joshi9 simplified. This will approximate the friction pressure loss problem into two categories: (1) oil flow into a horizontal well in a horizontal plane and (2) oil flow into a horizontal well in a vertical plane.
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