Microchannel Remediation of a Cement Packer Unlocks Mature-Field Potential
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Well RXY is located in Cairn’s Ravva offshore field in the Krishna-Godavari Basin in India. One goal for the field was significant crude production by means of a secondary reservoir section. This paper presents a case study concerning rigless remediation of microchannels in the cement packer (placed in the annulus of production tubing and casing to isolate the producing zone) and discusses laboratory development of a customized epoxy-resin system, simulations to estimate channel size, 3D displacement modeling, drillout after placement, and evaluation post-placement.
The well is an injector, drilled in 1998, intersecting several oil sands. The well was completed selectively across the sands with permanent packers for zonal isolation. Although several oil sands were intersected during drilling, one was not completed because of its marginal reserves. Because those sands fall above the production packer, a cement-packer job was attempted in 2016 to access the shallow sands while providing an annular barrier. However, after the job, communication was observed between the tubing and the annulus. The communication was attributed to poor cement isolation.
Estimation of Channel Size
Fluid-flow calculations and a hydraulic simulator were used to estimate the size of channels in the cement. The calculations were based on the real data of treated-water circulation. The circulation was established between the production tubing and the A annulus through holes punched at 2282 to 2284 m. For calculation purposes, the top of cement was assumed to be 1500 m (per the cement-bond log) and the average channel size from a depth of 2282 m to a depth of 1500 m was to be estimated. Also, the actual well directional data were used for the most-accurate hydraulics calculations.
After several iterations, the channel size was estimated to be 0.3875 in. in average thickness.
Various techniques were evaluated to remediate the issue of channeling and to restore zonal isolation. Because the scenario was identified as one involving a narrow cement channel to be treated by the pressure-balance method, a proprietary epoxy-resin system was selected as the sealant. The selection was made on the basis of the sealant’s ability to seal the microannuli behind the casing and to restore zonal isolation by shutting off the gas flow, its characteristics in developing high compressive strength, its ability to resist significant strain without failure, and its solids-free formulation.
Because of uncertainty regarding the size of the problematic flow channels, a fluid-train system was considered. The existing channels were to be blocked by an epoxy resin, which would be followed by an ultrafine cement system. The plan was for an epoxy-resin plug to be placed on top of the cement while the ultrafine cement filled the channels through the cement. The epoxy-resin system would also provide a barrier by forming a crosslinked polymer network within the porous media (microchannels). Ultrafine cement would help fill the larger voids and circulation holes to block the conduit between the tubing and the annulus.
Design of Epoxy-Resin System
These systems are designed such that the reactive epoxide groups in Resin 1 and Resin 2 are balanced with the active hydrogen groups of the amine hardener. An optional accelerator often is added to control the rate of reaction and decrease the reaction or thickening time; however, to maximize the time available to squeeze the resin into the perforations, no accelerator was used here.
For remediation of zonal isolation in tight spots or channels, solids-free epoxy-resin systems are desirable. These eliminate the risk of particle bridging and allow the resin to penetrate the zone to be treated deeply. With a base fluid density of approximately 9.2 lbm/gal, the fluid is naturally denser than water and many types of brine. Consequently, in this application, a weighting agent was not required.
Properties of Epoxy-Resin System. Rheology. The rheology of this system has two distinct features. There is essentially no yield point, and there is a linear relationship between stress and shear rate. These two features of the rheological behavior indicate that the liquid exhibits Newtonian fluid behavior.
Thickening Time. The initial consistency of the system is approximately 12 Bc. As temperature increased to 167°F, consistency reduced to 4 Bc. The initial point of departure occurred after 5 hours and 42 minutes, and consistency reached 100 Bc at 6 hours and 27 minutes. Unlike cement, the consistency of a resin increases gradually. Because resins exhibit Newtonian flow behavior, the limit of pumpability is generally referred to as the time to reach 100 Bc.
The bottomhole circulating temperature (BHCT) was estimated using dynamic temperature-modeling software that used selected parameters to calculate a reasonably accurate value. Furthermore, the value of the BHCT considered was based on the worst-case condition that the well has been static for a long duration and that this condition would yield maximum value. The BHCT was approximately 167°F.
Compressive Strength. The ultimate compressive strength of the epoxy-resin system was determined by use of a destructive test method for 2×2-in. cured cubes. This was undertaken because the values generated for inferred compressive strength of a pure resin system in an ultrasonic analyzer should not be taken as absolute, considering that the software algorithms are designed for cement; however, testing in this mode may be useful for suggesting the overall trend of strength development.
Design of Ultrafine-Cement System
In comparison with conventional oilfield cements, ultrafine cement particles are 5 to 10 times smaller and can be used effectively for penetrating narrow channels. Generally, ultrafine-cement slurries are lightweight (11.0 to 12.5 lbm/gal) because of the high water concentrations required to wet the extensive surface area of the dry cement fully. Because of its reactive nature, early compressive strengths of ultrafine cement are highly relative to slurry density.
Penetration performance, the capability to invade a restrictive opening, is the key attribute that must be considered in the design of an ultrafine-cement slurry. Once penetration into the narrow openings has occurred, the slurry must harden to form a permanent seal.
2D Hydraulic Calculations and Design Simulation
Various simulations were run to examine hydraulics, fluid displacement, and other parameters during placement and mechanical performance of the sheath. The simulation software enabled a better prediction of the top of the cement, the fluid-intermingling effect on cement purity, and the extent of mud channeling. It also enabled prediction of required material choices and volumes, potentially reducing the cost of an operation. Such specific investigative modeling helps improve well economics, with the annular barrier helping increase ultimate recovery.
3D Displacement Efficiency
Finite-element analysis of 3D displacement efficiency was performed with the help of software, with prognostic models simulating fluid-flow interaction, displacement phenomena, fluid-intermingling effects on cement purity, and the extent of mud channeling. The volume and rate of the fluids pumped were simulated for different scenarios to achieve the primary goal of effective zonal isolation. For the analysis of a measured depth of 2060 m (the target-reservoir zone), fluids present in the concentration included inhibited water (1.2%), packer fluid (22.7%), epoxy resin (5.9%), and ultrafine cement (70.1%). The fluids were mixed in the percentages mentioned, and tests for pumping time and compressive strengths were performed.
For a measured depth of 2284 m (circulation holes in production tubing), the same fluids were present but at percentages of 0.4, 18.0, 5.3, and 76.3, respectively. The fluids were mixed in the percentages mentioned and tests for pumping time and compressive strengths were performed.
A range of scenarios, permutations, and combinations was evaluated, and final pumping volumes were chosen in order to have at least a 300-m isolation plug above the zone of interest. Furthermore, a worst-case scenario was evaluated to select volumes such that a 30-m good isolation above the zone of interest could be achieved even in the case of failure to pump the complete remedial train.
After the job, the well was pressure-tested to check the integrity of the tubing. Later, the cement-bond log, using an ultrasonic tool, was conducted to confirm the zonal isolation. An earlier baseline cement-bond long was superimposed on the cement-bond log conducted after the remedial job, and an improvement in cement isolation was clearly seen. The ultrasonic scanner identified the epoxy resin successfully and provided acoustic-impedance values within tolerance on the basis of acoustic laboratory testing of resin samples. Furthermore, acoustic impedance was post-processed to generate a derivative acoustic-impedance data set before following the data-analysis work flow.
The well was perforated at the desired zone of interest and successfully began production at the initial rate of 2,000 BOPD. This result shows that tight leaks and microcement channels in a production annulus can be treated effectively by rigless means and by pressure-balanced placement using the epoxy-resin system. This case can be used as a reference for future remedial works in which the cement or resin is supposed to be used as a secondary packer. Thus, high compressive strength, fluid immiscibility, elastic nature, and solids-free-formulation make resin part of an effective solution for restoring zonal isolation. The additional oil production added significant value to this field.
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Microchannel Remediation of a Cement Packer Unlocks Mature-Field Potential
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