RIASSUNTO
Abstract
Chevron has developed a number of new techniques for abandoning wells with severe parted casing, which have been successfully applied in an unconventional Californian reservoir. A number of wells have developed severe doglegs or parted casing. Such restrictions have prevented traditional workover techniques from reaching the reservoir to ensure a standard abandonment, referred to as ‘complex damage'. The first key learning is to maintain access to the reservoir as long as possible, by not pulling the equipment (tubing and packer) out of the wellbore when parted casing is suspected. In cases where it may not be possible to abandon the well after equipment is pulled from the well-bore, solidly cementing the equipment in the well may be a more robust abandonment technique than pulling the equipment and risking not being able to re-access the reservoir. When complex damage has developed, milling and explosive tools have been successfully applied to re-access the reservoir, run on tubing, wireline and coiled tubing. To evaluate the restrictions, diagnostics such as down-hole cameras are critical to develop and execute a successful abandonment plan. Using these techniques, Chevron abandoned several wells with complex damage in 2011.
Introduction
A group of damaged wells in Chevron's unconventional Californian reservoirs were found to require new methods to enable abandonment. A subset of these wells has restrictions so severe that the reservoir is no longer accessible using traditional workover methods. In a number of these wells the casing has sheared and is offset such that the lower portion of the wellbore cannot be re-entered using 2-7/8?? tubing or 1-¼?? coiled tubing. Furthermore, the 2-7/8?? tubing string has also parted in some wells, and possibly leaving a fish below parted casing, where fishing tools are not able to access. Chevron has termed wells with such severe restrictions as ‘complex damaged wells'. In 2011 Chevron commissioned a project to develop techniques for competently abandoning complex damaged wells, which used dedicated petroleum engineering and drilling & completions resources, along with engaging various down-hole vendors with specialized workover and fishing expertise. A number of new techniques have been developed to successfully abandon wells, which will be outlined in this paper.
Reservoir Overview
The unconventional Californian reservoir in which these techniques have been applied by Chevron is a late Miocene reservoir within the Antelope shale of the Monterey Formation. The reservoir is an erosionally truncated, northwest-southeast, doubly plunging anticline. Lithologically it consists of primarily clean to sandy siliceous shale, with lesser components of dolomitic shale and sandstone consistent with a deep marine depositional environment. Heavy oil is produced from the low permeability, high porosity reservoir. The reservoir is bound above by the Pliocene/Miocene (P/M) unconformity and by the diagenetic Opal CT transition at its base. The P/M unconformity averages ~975' below surface; the reservoir is approximately 435' thick, covering ~400 acres. Structurally, reservoir strata dip more steeply to the northeast than on the southwestern limb of the fold, with upper strata truncated by the P/M as they approach the crest of the anticline.