Abstract

The unconventional shallow gas market is the fastest growing oil and gas sector in Western Canada. Although these gas reserves are enormous, the economic output of a single well is marginal. The low production rate of each well has forced operators to reduce costs wherever possible. The largest expense in the completion of a well is stimulation. The advent of the coiled tubing fracturing method has reduced these costs substantially, as all zones can be individually isolated and stimulated in one operation.

Operators are taking further action by drilling multiple slanted wells from a single surface location. This configuration requires only one lease, one access road and reduces production costs with a single pipeline collection point. Workover equipment with the ability to rig onto these wellheads safely is in short supply. Many shallow gas wells utilize threaded wellheads that are not designed to support sideloads. Coiled tubing equipment is relatively massive, and previous attempts to 'block up' the wellhead and blowout preventers (BOPs) have resulted in near-tragic consequences. To overcome these obstacles, a patented support system capable of safe and efficient rig-up of BOPs on slanted wellheads was designed and built. The support system can be rigged onto wellhead angles ranging from 30–95 degrees. The system can manipulate the BOPs in virtually all planes to allow for flexibility when positioning onto a wellhead. The weight of the BOPs and CT equipment are then buttressed by the support system, and not the wellhead. The development of this system has enabled safe coiled tubing fracturing within the economic and technical contraints of shallow gas and coalbed methane type fields.

Introduction

The coiled tubing fracturing technique has dramatically reduced the cost of completing low-production, multi-zone wells, such as those found in the shallow gas market of western Canada and the western United States. After a wellbore is drilled and cased, all zones of interest are perforated in one trip with a wireline unit. To stimulate the well, a coiled tubing fracturing tool is attached to the end of large diameter coiled tubing (Figure 1). The tool has a fracturing port located between two neoprene or urethane cups, which act as packoffs to isolate the zone of interest. The straddle between the cups can be adjusted by adding pup joints to accommodate zones of virtually any size. Over 25 zones can then be individually isolated and stimulated in one trip to the well.

The shallow gas market in Alberta requires drilling a high number of wells to fully exploit a play. A typical square mile (2.6 km2) section of land will usually contain 6–8 wells. For every lease, operators must pay initial access rights fees and annual rent to the landowner. A large portion of the well cost in the shallow gas market over the economic life of a well is the result of land access fees. The enormous volume of wells, coupled with the increasing cost for land access rights, has prompted many operators to consider slant well pad drilling (Figure 2). The slant well pad configuration requires only one lease, one access road and reduces production costs with a single pipeline collection point. The operating company is only required to pay the landowner access fees and rent for one lease only. A slant well pad will also reduce construction and production costs, as all wells are concentrated in one area.

Early attempts at coiled tubing fracturing on slant wells were conducted using two cranes to rig the BOPs onto the wellhead. Once the BOPs were attached, various items were used to attempt to 'block up' the wellhead, including coiled tubing injector legs and wooden planks (Figure 3). Once the wellhead was 'supported', one crane was removed from the BOPs to assist a third crane in lifting and tilting the coiled tubing injector.

Incidents have occurred during coiled tubing fracturing operations due to excessive sideloading of the wellhead. One incident occurred while removing the BOPs from the slanted wellhead after a fracturing treatment. A pup joint threaded onto the casing failed at the threaded connection below ground. The root cause of the incident was determined to be inconsistent support of the weight of the BOPs while rigging on and off the well with two cranes. Similar incidents were repeated across the industry while using cranes to support the BOPs on slanted wellheads. Operators were forced to re-evaluate the feasibility of drilling slant wells due to the limited service equipment available and the inherent difficulty of completing these wells.

This content is only available via PDF.
You can access this article if you purchase or spend a download.