Abstract

For a company operating in a mature province, it is extremely important to continuously seek new ways to improve recovery from known hydrocarbon accumulations. As the mechanical process of drilling horizontal wells matured in the mid-1990’s, Shell Western E&P Inc. (SWEPI) was quick to apply this new technology to just such a province, the Northern Silurian Niagaran Pinnacle Reef Trend (NRT) in the Michigan Basin. To date, SWEPI has drilled more than 60 horizontal wells in this trend, which accounts for over 55% of all horizontal drilling in the NRT, and continues to be the industry leader. Much has been learned over the past decade, and it appears that the application of lateral drain-hole technology can be an effective tool in the NRT in the right geological and reservoir situations.

SWEPI first attempted to apply horizontal technology to the NRT in 1987. These early attempts failed mechanically due to the inability to maintain a horizontal path through a significant section of reef. In only one well, the Bancroft-Bisard 2-1, was a horizontal path achieved, and that lateral was less than 100 feet in length. No further horizontal activity occurred until 1994, when a review of the decline curve from this well indicated that not only had there been a production increase of 30 BOPD after the lateral, but the rate of production decline had been reduced from 37% to 18%. It was estimated an additional 100 MBO would be recovered from this well as a result of the lateral. This information, coupled with improvements in drilling technology, and the knowledge that great improvements in production were being made world-wide by the application of lateral technology, caused SWEPI to initiate a new lateral program in the NRT.

There were five major opportunity types evaluated with horizontal wells. Our first attempt in 1987 was a large gas cap over a thin oil leg. This type has continued to provide the most consistently positive results, and accounts for 47% of the total laterals drilled. The second largest opportunity type with 36% of the total is the heterogeneous reservoirs. These opportunities closely resemble exploratory drilling where the success rate is low but the risk capacity is high. To date, only two low-pressure reservoirs, which depend on pure gravity drainage, have been drilled. Neither was successful. Another type is the improved kh in a low porosity reservoir. While SWEPI has drilled only four, these can be profitable in certain circumstances. The most disappointing type has been the thin oil column over a large water leg. We have drilled three thin oil column horizontals with no successes.

Approximately 40% of the total 55 horizontal wells drilled by May 1998 were considered economically successful. Ultimately, SWEPI has forecasted that the horizontals will recover an additional 3.4 MMSTBO and 5.7 BCFG.

Issues that remain a concern include lost returns, wellbore damage, vertical position of the lateral, and the application of laterals in settings that have been unsuccessful to date. Significant oil volumes remain in these environments. Attempts are being made to address some of these issues as the program continues to mature.