RIASSUNTO
Abstract
Subsidence Induced Casing Doglegs (SICDogs) occur in many oil reservoirs worldwide. Doglegging, at a minimum, leads to reduced mean time between failures of rod pumped wells. SICDog remediation techniques in the past have had limited success and are relatively expensive procedures. Most of these techniques have used various rotating mill assemblies and different strengths, sizes and connections of drill pipe. Some techniques have been extreme, such as destroying the dogleg with explosives. A multiphase, risk-sharing, team effort over four years between Aera Energy LLC and Weatherford International Ltd led to the development by Weatherford of a hybrid broaching/milling tool. This hybrid milling system reliably and quickly remediates multiple doglegs in a single well bore. Seven Diatomite reservoir producers in the South Belridge Oil Field, Kern County, California were repaired and successfully returned to production in the latest four well phase. The work was routine in nature, requiring no special or extraordinary services. In this paper, we will discuss the geometry of SICDogs and the challenges they pose to rotational milling. Other topics will include the evolution of the hybrid tool from experiences in this and other projects, operational concerns such as well control and repair options of the remediated SICDog. Lessons learned during the evolution of this technique will be discussed at length.
Introduction
Casing doglegs are just one of many forms of casing deformation created by shifting in the rock surrounding the well casing (see Figure 1). Reservoir compaction and subsidence can lead to secondary horizontal motion along planes of weakness, such as bedding planes, that leads to casing shear[1]. Casing shear causes the formation of Subsidence-Induced Casing Doglegs (SICDogs). SICDogs in rod pumped wells lead to reduced mean time between failures (MTBF) due to the increased friction between rods and tubing through and near the dogleg. Common mechanical failures are rod parts and tubing holes at SICDog depths. There is a direct correlation between SICDog severity and reduced MTBF.
Eventually it will become uneconomic or simply impossible to rod pump the well due to the inability to run equipment through the dogleg without bending or breaking the equipment. The candidates in this study are wells that have SICDog deformations that are severe enough to bend or corkscrew 2 7/8?? tubing pulled through the deviation. Like a pipe bending machine, the dogleg causes plastic deformation of the metal.
There are four solutions to SICDogs
Prevention of SICDogs.
Circumvention of SICDogs, such as a horizontal well drilled from outside the shear zone.
Replace the damaged wells.
Remediate the damaged wells.
Only SICDog Remediation will be covered in this paper.
History of SICDog Remediation
Efforts to categorize and measure casing doglegs appear in the literature in the early 1990's. Subsequently, different types of subsidence-induced deformation were noted, such as compressional buckling and the dogleg category due to casing shear[2]. There was some reported activity to monitor and remediate doglegs in the early stages of dogleg formation before the dogleg began to reduce wellbore utility[3]. Efforts to further characterize and remediate doglegs appear in the literature later[4]. On relatively minor dogleg deformations, conventional rotating mills were used in the Lost Hills and Belridge oil fields, Kern County, California [5]. Some innovations were made with newly designed specialty mills. There were anecdotal accounts of limited success in both fields. Explosives were even used at one point in the early 2000's to essentially destroy the doglegs, creating a cavity[5]. A fire hose filled with an explosive mixture was detonated after running it inside the dogleg. After the dogleg removal, an expandable casing patch was intended to restore casing integrity across the annihilated dogleg interval. There was limited success here also.