Optimization of Well Length for Horizontal Drilling
Authors: Cho H. , Shah S.N. 
Published: March 2010

As the length of a horizontal well is increased, its drainage area also increases. Productivity of a long horizontal well is no longer proportional to the well length, because an increase in the length of the horizontal production section also causes frictional losses in the wellbore. However, no reliable standards are currently available that take into account quantitatively the parameters necessary for the determination of an optimum well length for horizontal drilling.

A new approach of the specific productivity index is proposed to predict the production rate, considering friction losses in a long production section under inflow conditions. This specific productivity index is derived by assuming steady-state flow of a slightly compressible fluid in an anisotropic formation. This paper presents a methodology developed to obtain the optimum horizontal well length based on total economics as well as 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, in order to achieve the overall optimum performance of a horizontal well project.

Introduction

One of the advantages of a horizontal well is to achieve a large reservoir contact area. 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 increase in frictional losses. Recent experience with horizontal wells has revealed that, in many circumstances, the inflow performance of horizontal wells does not match the expected productivity, and that 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 productivity index is no longer directly proportional to the well length.

A survey(3) 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 being economically successful(3). Another point in horizontal drilling is the incremental expense over conventional drilling. It costs about 1.4 to 3 times(4–7) more to drill a horizontal well, than a vertical well depending, upon the drilling method and completion technique employed.

Another horizontal well survey(6) 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 over 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?