Performance Evaluation of Horizontal Wells in a Tight Carbonate Reservoir
Authors: Kabir C.S. , Del Signore A.G. , Al-Fares A.A. 
Published: December 2007

This paper presents a case study of evaluation of the performance of horizontal wells drilled in a tight carbonate reservoir in Kuwait. Successful wells were those that intercepted natural fractures. A methodology was developed to define the reasons for the performance of horizontal wells. The methodology involved use of geological, petrophysical geochemical, transient test, rock, and fluid properties to develop a numeric simulation model. Conclusions regarding well performance were made by using a newly developed analytic simulator and the numeric model.

While the analytic simulator provided rapid rate forecasts, the numeric simulator gave insights into the causes for unsatisfactory well performance. For example, well placements within the bed, low-permeability rock, and unfavorable relative permeability characteristics all contribute to the rapid rate decline and the associated increasing gas/oil ratio (GOR) production. We proposed cyclical production and oil injection-production schemes to address the high-GOR production problems. In the first scheme, a well is produced and shut-in in an alternating fashion to preserve the reservoir energy. Oil is injected for a very short duration to alleviate the near-wellbore gas saturation problem in the second scheme. In both cases, the high-GOR production is minimized and the ultimate oil recovery is improved.

Introduction

Horizontal well technology has evolved rapidly since pioneering drilling efforts were begun in late 1970's in the Raspo Mare Field offshore Italy. In the US, spawned by successes at the Austin Chalk formation in East Texas since 1985, the horizontal drilling activity gained considerable momentum. Austin Chalk 1 happens to be a tight carbonate with extensive vertical and sub-vertical fracturing. Despite publication of numerous successful case histories, Beliveau, while compiling a major operator's world-wide horizontal well efforts, observed that many-fold production increases do not often materialize because of incomplete knowledge of reservoir heterogeneity before drilling. North Sea's Ness field is also a case in point.

Combined industry experiences suggest that the success of horizontal wells lie in selecting the right candidate reservoir. Fayers et al. summarize some of the reservoir engineering issues that need considering while evaluating horizontal well prospects.

Besides gaining productivity increases in general, horizontal wells have enjoyed successes in combating coning or cresting of water and/or gas, in accessing by-passed oil and multiple flow units, and in converting resource into reserves, such as those in heavy oils and tight reservoirs. Examples of high-permeability sandstone reservoirs where water coning has been mitigated include Troll field in the North Sea, Prudhoe Bay field in Alaska, Safaniya field in the Middle East, and those in offshore west Australia. Similarly, horizontal wells in Rospo Mare, the Karsted limestone reservoir, have enjoyed considerable success in mitigating water coning. While horizontal well successes are often presented in various forums, the associated failures do not gain as much publicity. Generally speaking, occurrences of failure are high with carbonate reservoirs where successes depend largely upon horizontal wells intercepting natural fractures. South Kuwait's Mauddud reservoir in discussion is a low-permeability carbonate with sparse fracture development. Successful exploitation of reserves with horizontal wells in similar reservoirs has not been reported. Only the Dan field in the North Sea and the Kharaib B reservoir offshore Qatar have permeabilities that are comparable to this reservoir; that is, 1 to 10 md. However, they are much thicker and lend themselves to hydraulic fracturing because of confining shale barriers. Besides accessing fractures, the fluid-rock interaction with changing fluid saturations plays a critical role in the success of long-term producibility potential of a well. P. 643^