Abstract

Underbalanced Drilling (UBD) of Horizontal wells is gaining popularity in Western Canada. The technique has been used to drill more than 250 wells in the last two years. Advances in drilling equipment and drilling techniques are mainly responsible for the progress of this new evolving technology. The advent of rotating BOP's and " Closed-System" surface facilities to handle drilling fluids and produced hydrocarbons has greatly increased the safety and efficiency of the drilling operation. Many of the " short term" benefits of UBD, such as increased penetration rate, and evaluation of the productive zone while drilling, have been well documented. However, some of the " long term" benefits are not clearly defined. This paper presents a study conducted on 100 horizontal wells drilled in the Midale, Frobisher-Alida, and Tilston formations in SE Saskatchewan. About half of the wells were drilled overbalanced and the others were drilled underbalanced. The objective of the study was to examine the initial stabilized production rate, the productivity index (PI) and the mechanical skin. These parameters were used to provide a better appreciation of the " long term" benefits of UBD. The results indicated that the initial production rate of the UBD wells was generally superior to wells drilled overbalanced. However, the other two parameters did not clearly indicate this superiority.

Introduction

Horizontal Well Technology (HWT) is evolving at a brisk yet cautious pace. During the last five years, about 2500 horizontal wells have been drilled in Western Canada. The number of horizontal wells drilled in elastic and carbonate formations are almost evenly divided. Typical true vertical depths (TVD) drilled during the 1989–91 period were 500 to 1000 meters. The horizontal lateral of these wells were also in the 500 to 1000 meter range, resulting in a " measured well depth" of 1200 to 2000 meters. Several horizontal wells, especially in Alberta and British Columbia are now being drilled in formations at depths of 2000 to 2500 meters.

Also, in the early stages of the technology most wells were " New Drills," using conventional mud systems. Today, at least 25% of horizontal wells drilled may be classified as " Advanced Technology" HWT projects. These include Underbalanced Drilling (UBD) using nitrogen, natural (methane) gas or air; Re-Entries (RE)from 114.3 to 177.8mm production casing; Short-Radius (SR); 0 to 90 degrees in 12 to 20 meters; and Multi-Laterals (MI.); one or more arms radiating from a main lateral. (Maurer et al 1994). Some production data is becoming available for wells using these

" Advanced Technology," and we are now able to examine some of the " long term" benefits of these techniques. In this paper, we will look at some of the " short term " and " long term" performance benefits of UBD. In particular, we will discuss a study conducted on horizontal wells drilled in SE Saskatchewan which examined the performance of horizontal wells drilled in the Midale, Frobisher/Alida, and Tilston formations. (More than 100 horizontal UBD wells have been drilled in these formations) wells reviewed in the study. The table indicates drill time, initial production rates, actual productivity indices, total alculated skin, and whether the wells were drilled overbalanced or underbalanced.

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