Drilling horizontal infill wells in the Pierce field in the UK central North Sea is challenging because of a narrow drilling window caused by depletion in a highly fractured reservoir. Wellbore strengthening was attempted in the reservoir section of Pierce B5 although, when a pre-existing fracture further weakened by depletion was encountered, losses occurred. A detailed analysis of the losses event on Pierce B5 provided an improved understanding of the loss mechanism, resulting in a revised equivalent-circulating-density (ECD)-management strategy.
Introduction
The Pierce field (Fig. 1) in the UK central North Sea is a brownfield development with 17 existing wells drilled around two complex salt diapirs. Oil production was depletion-driven for the first 7 years until pressure support was provided by three water injectors drilled around the southern salt diapir. Pierce B5 was drilled in the proximity of the southern diapir, and losses encountered in the reservoir section were severe enough that no further drilling progress could be made. Pre-existing fractures further weakened by depletion were thought to be the cause of the losses. Unfortunately, from a well-planning perspective, these fractures cannot be predicted or avoided when drilling horizontal wells.
During a three-well infill campaign (Wells B5, A11, and C1), considerable losses on the first well led the operator to re-evaluate how future wells would be drilled. The outcome of this was the development of a low-ECD-drilling-fluid system, a revised ECD-management procedure, and a much-improved loss-treatment strategy. This combined strategy was applied in the field and resulted in the successful drilling of the subsequent wells.
Loss Event on Pierce B5
Because of the close proximity in the reservoir of the Pierce B5 well to a pre-existing production well (Pierce B2), it was determined that the localized depletion seen at this location because of the drawdown effects of B2 would be on the order of 2,300 psi, which would result in a reduction in formation integrity. However, there was concern that the effects of depletion could be higher than that; 3,200 psi through open fractures was modeled as the worst case. Therefore, because of the potential for dynamic losses when encountering zones that have seen the lowest field depletion and to mitigate the risk of open fractures, the recommendation was made to use a fluid design incorporating wellbore-strengthening materials (WSMs).
Drilling the first 2,066 ft of reservoir out of a planned total of 2,357 ft exposed the wellbore to stable ECDs of 12.48 lbm/gal without any induced losses, which suggested that wellbore strengthening had been effective in providing an additional drilling margin. However, at this point in the well, losses occurred both dynamically (while pumping) and statically, with initial loss rates of 200 and 65 bbl/hr, respectively. These losses occurred shortly after a connection was made and an ECD spike of 12.67 lbm/gal was observed; drilling then continued for the next hour with a decreasing trend in the drilling ECD, which was 12.38 lbm/gal when losses occurred. The ECD trend demonstrated that there was no ECD spike or warning of losses when they occurred, suggesting that drilling into an existing weakened zone or fracture was the cause of the losses.
Over the next 10 days, more than 5,000 bbl of nonaqueous fluid (NAF) was lost downhole and 15 individual lost-circulation-material (LCM) treatments were performed in an attempt to halt the losses. More than 51 t of LCM was used without success before the well stabilized enough to secure the drilled footage and complete the well. Two main types of LCM treatments were used: (1) a combination of predominantly fibrous materials, ground nut shells, and metamorphized limestone (marble), designated as Pill B; and (2) a thermally activated gelling agent for NAF. It was suspected that the losses were attributable to a pre-existing fracture or fractures with a fracture width exceeding what the WSM was designed to bridge.
Wellbore Strengthening on Pierce B5
Wellbore strengthening was chosen for B5 because of concerns about localized depletion around B5. Optimum WSM treatments were derived using algorithms for particle-size distribution and fracture sealing. Operationally, the material was added as a concentrated slurry at the rigsite at a controlled rate to keep the coarser particles in the circulating system.
Pierce A11: Low-ECD Drilling Fluid and Losses Strategy
Planning for Pierce A11 was taking place when the losses on B5 occurred. A11 was clearly a challenging well, and additional focus was drawn to the design team following the losses on B5 to accomplish the following goals:
- Decide if wellbore strengthening was necessary for the successful drilling of A11.
- Develop a strategy to reduce ECD while drilling, through the development of a low-ECD drilling fluid based on a high oil/water ratio (OWR).
- Design a robust LCM strategy on the basis of the best products currently available for the region to cater to large fractures.
Low-ECD Drilling Fluid
The use of WSM on B5 was far from basic, so with a larger drilling window available for A11, the opportunity existed to drill the planned 7,300 ft of reservoir conventionally. The key was ECD management, where drilling-fluid properties and drilling parameters were monitored closely to ensure the ECD was kept as low as possible. A successful system is one in which its low rheological profile (low viscosity) leads to a reduction in ECD while drilling but does not affect the drilling-fluid cuttings-carrying capacity or increase the likelihood of barite sag.
The solution on the Pierce wells to achieving a low-ECD-drilling-fluid system together with high barite-sag resistance is in the OWR, coupled with an optimum organophilic-clay content. In NAFs using organophilic clay for viscosity, the concentration of organophilic clay will increase as the OWR increases in order to maintain a particular rheological requirement. This is because the organophilic clay will yield less as the OWR increases. Organophilic clay can be thought of as the backbone of an NAF for reducing the likelihood of barite sag when used in sufficiently high concentrations.
The OWR range programmed for A11 was from 75:25 to 85:15. The drilling-fluids engineers were encouraged to aim for the higher end of the range but allow for dilution with base oil to control solids toward the end of the section. Limits for both ECD and rate of penetration (ROP) were set to stay within the expected formation-integrity measurements; the ECD limit was set at 12.5 lbm/gal, and an ROP limit was set at 50 ft/hr. The offshore team was made aware of the importance of ECD management. This is higher than the possible real low value for formation-integrity measurements; however, fracture breakdown pressure will exist even with depletion. The amount of additional strength is unknown and will be lost if ECDs are too high or a fracture is encountered. The result would be the formation strength equaling the formation-integrity measurements, and losses occurring. The key to lowering the induced-losses risk, therefore, was to limit ECD as much as possible, adhere to good drilling practices, and use the low-ECD drilling fluid.
The decision to use the low-ECD drilling fluid (weighted with ultrafine barite) was the result of modeling predicted ECDs.
LCM Strategy. The operator’s well-fluids team evaluated possible LCM treatments from incumbent fluid vendors that might enable the sealing of large fractures (>3 mm). The following LCM solutions were proposed for Pierce A11:
- High-fluid-loss/high-strength (HF/HS) pill.
- Soft-setting cement plugs with additives to prevent accidental sidetracking (reduced compressive strength).
- Specially selected synthetic fibers originally designed for cement spacers.
- Shear-thinning water-based mixed-metal-oxide fluid treatment.
- Pill A and Pill B—LCM blends including fibrous materials, ground nut shells, and marble. (Pill A represents a light treatment, and Pill B represents a heavy treatment.)
The LCMs chosen for Pierce A11 were Pill A, Pill B, and the HF/HS treatment.
HF/HS Functionality
HF/HS is a one-sack inert multifiber lost-circulation pill treatment designed to be easily mixable in a variety of media and engineered to defluidize rapidly within the loss zone, leaving behind a resilient, high-solids plug in a fibrous-lattice form (Fig. 2 above). As the filtrate is squeezed into a permeable formation and the consolidating matrix of solids increases in thickness, its resistance to differential pressure and mechanical force also increases. Pill density can be controlled either with a weighting agent such as barite or through the use of a brine base, as in Pierce, to help maximize shear strength.
Results From Pierce A11
On A11, 2,169 ft of pilot hole and 4,608 ft of main bore were drilled by use of the revised ECD-management and low-ECD-drilling-fluid strategies. The ECD throughout both hole sections was maintained below the 12.5-lbm/gal ECD limit. One instance of losses occurred while drilling the pilot hole when crossing a fault with a 12.15-lbm/gal ECD, but this cured itself and no LCM treatment had to be used. No further loss events occurred, with both hole sections being delivered successfully as per the plan.
Pierce C1 Planning
Pierce C1 was the 17th development well drilled in the Pierce field. It consisted of a 2,850-ft horizontal reservoir section targeting the Forties Paleocene reservoir and was planned as the most northerly production well at the top of the North Pierce diapir.
The available drilling window was assessed, and a low-ECD-drilling-fluid, ECD-management, and LCM strategy was planned to drill the well. ECD management would be critical, and the following lessons from A11 were introduced to C1:
- Establish optimum OWR with laboratory testing on the low-ECD drilling fluid.
- Control ROP to a 50-ft/hr average value and a 70- to 80-ft/hr instantaneous value.
- Limit ECD to 12.30 lbm/gal.
- Track pressure-while-drilling data and hole cleaning in real time.
- Optimize marble additions to the drilling fluid (added in case of stuck pipe).
- Establish a testing method to measure marble concentration in an NAF.
- Adopt narrow-margin drilling-window practices (stage up pumps, reduce surge pressures).
- Provide additional wiping of the hole on connections to improve hole cleaning as per A11.
- Use a minimum of 150 rev/min when drilling the reservoir section.
Pierce C1 Execution, Losses, and Application of the HF/HS Pill to Halt Losses
Pill A was prepared and pumped and allowed to soak for 50 minutes. Attempts were made to achieve a drilling loss-free flow rate; however, even at a lower flow rate of 225 gal/min, the loss rate was 60 bbl/hr. This high loss rate at such a low pump rate was considered too high to continue drilling. The decision was made to pump an HF/HS pill. To help maximize product shear strength, the HF/HS pill was prepared using 10-lbm/gal sodium chloride brine. This was then increased to 11.28 lbm/gal with barite. Subsequently, 20 bbl was pumped downhole with the bit 3 ft off-bottom.
Circulation was then established with 24-bbl losses at 450 gal/min, which decreased quickly to loss-free status. On the basis of this positive outcome, the decision was made to drill ahead, increasing flow rate to 488 gal/min. Losses initially were as high as 60 bbl/hr but reduced to a sustainable 8 bbl/hr. For the next 2 days following the HF/HS-pill treatment, no additional LCM treatments were made to the drilling-fluid system.
The 7×5½-in. liner was run and cemented successfully. Of the 188 bbl of cement pumped, 22 bbl was lost to the formation and 7 bbl of cement was circulated from above the hanger after the cement job, indicating that a complete cementation of the liner had been achieved.
Discussion
Evidence of pre-existing fractures was captured by the neutron-density-logging tool on Pierce C1 and, therefore, validates the theory that losses on B5 and, subsequently, C1 were caused by pre-existing fractures weakened by depletion. Because losses of this nature are a recent phenomenon in Pierce, this would suggest that increasing the reservoir pressure in the field, from the lows before water injection, has not improved the overall strength of the formations encompassing these fractures. On B5, the risk of losses was exacerbated further by the localized depletion from the nearby B2 well. This implies that future drilling around the Pierce diapirs may also suffer losses; however, the likelihood of predicting pre-existing fractures that would be problematic is low because many faults were crossed without any issues.
The HF/HS pill has been shown to be an effective LCM where losses occur in pre-existing fractures. The ability to apply a single effective treatment to cure severe losses is a huge advantage when planning depleted wells. The HF/HS pill can be recommended for loss zones similar to those in Pierce C1 or B5, in fractured and depleted sandstone reservoirs.
This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 166134, “Case Study—ECD-Management Strategy Solves Lost-Circulation Issues in Complex Salt Diapirs/Paleocene Reservoir,” by David Murray, Shell; Mark W. Sanders, SPE, and Kirsty Houston, SPE, M-I Swaco; and Hamish Hogg and Graeme Wylie, Shell, prepared for the 2013 SPE Annual Technical Conference and Exhibition, New Orleans, 30 September–2 October. The paper has not been peer reviewed.