The industry’s push into new frontiers and increasingly difficult formations is driving different ways of thinking about wellbore instability, one of the leading causes of nonproductive time (NPT) and expensive intervention operations due to lost circulation and stuck pipe. Drilling depleted or weak zones with narrow operational windows has long vexed producers, but maturing fields, extended-reach wells, complex well designs and the move into deeper fields have compounded the problem, raising the bar for solutions that go beyond conventional methods.

The Gulf of Mexico (GoM) is among the sectors where maturing fields, characterized by depleted sands and underbalanced conditions, have put wellbore instability issues at the top of the list of operator concerns. For example, the GoM’s deeper plays hold a wealth of hydrocarbons, but reaching the target often requires drilling through the depleted zones on top, where mud losses can be significant. In this high-risk, high-cost arena, operators need innovative thinking with services that provide a comprehensive, integrated approach to problem-solving.

Historically, the industry has addressed wellbore instability by pumping lost circulation material (LCM) such as pills or cement squeezes downhole. In today’s challenging environments, however, these approaches have had limited success, primarily because they are reactionary rather than preventive.

Over the last decade, the concept of wellbore strengthening (WBS) has evolved, following a joint industry project (JIP) on WBS and lost circulation undertaken by M-I SWACO and several major operators.

With the focus on WBS, the project led to the development of loss prevention materials (LPM), a new class of products aimed at preventing downhole fluid losses vs. the conventional LCM methodologies that were designed to remediate losses. This proactive approach further resulted in the emergence of a new technology, fracture propagation resistance (FPR), which fortifies the wellbore.

The technique actually uses the fractures as part of the strengthening strategy by continuously applying various LPMs, or WBS additives, at low concentrations while drilling, then circulating the material for recycling and reuse. FPR has been shown to achieve greater effectiveness in nonpermeable formations than previously used methods, including stress cage and fracture closure stress.

The FPR process does not necessarily prevent fracturing but rather seals and isolates fractures from further propagation by creating a barrier for pressure transmission inside the fracture and effectively raising the fracture gradient. The approach maintains the required levels of LPM in the drilling fluid, providing constant protection while intersecting depleted and weak zones. This allows operators to construct wells in low fracture-gradient zones, avoiding costly losses, potential stuck pipe and sidetracking jobs while reducing drilling costs and NPT.

Integrated approach to WBS

FPR is a key feature of the M-I SWACO I-BOSS integrated borehole strengthening solutions portfolio that provides a multidisciplinary approach to solving wellbore instability issues. By combining predictive engineering software for planning, testing and monitoring; expertise; and advanced fluid systems and other products under one umbrella, the suite provides a comprehensive workflow to borehole strengthening. The workflow has been applied successfully in the GoM, reducing downhole mud losses by more than 80% in difficult deepwater operations impacted by depleted sand sections and zones with narrow pore-pressure/fracture-gradient drilling windows. For one operator, the approach was significant in removing the cost of lost circulation incidents from the top 10 NPT contributors.

The comprehensive workflow was awarded E&P’s Special Meritorious Award for Engineering Innovation at the 2009 Offshore Technology Conference. The approach has since been advanced to help operators further reduce mud losses and NPT in challenging wellbores.

An important component of the multidisciplinary workflow is the OPTI-STRESS software, which uses conventional hydraulic fracture theory along with the expected stresses and pressures of the formation, rock properties and drilling conditions to predict the most likely fracture size caused by the overbalance between the wellbore pressure and the fracture gradient. The software recommends the type and concentration of LPMs required to seal the fractures in a specific well, and this recommendation is based on findings from a comprehensive JIP involving more than a dozen operators and anchored by M-I SWACO. Those factors, along with the size and particle size distribution of the material, are crucial in creating an optimal WBS blend.

Higher concentrations of LPM additives can be present in the drilling mud because the materials are continuously recycled and reintroduced using a managed particle size recovery system (MPSRS), which is composed of solids-control equipment to remove cuttings and low-gravity solids while recovering the LPM at sizes that effectively raise the FPR.

The workflow also incorporates the newest generation of a flat-rheology drilling fluid system, the RHEALIANT PLUS synthetic-based fluid system, which maintains optimum viscosity with temperature fluctuations that commonly occur in this HP/HT environment. Invert-emulsion drilling systems, including environmentally friendly synthetic-based muds (SBMs), provide greater stability and lubricity in the reactive shales of the GoM and also perform better for extended-reach and high-angle drilling.

The FPR technique, in conjunction with MPSRS, was applied successfully by a major operator in several subsea development wells in a challenging field in the GoM. The wells, located in the Greater Mars-Ursa Field of the Mississippi Canyon in water depths of more than 914.4 m (3,000 ft), had been encountering significant losses while drilling the depleted sand sections.

Reaching GoM targets

The first group of wells was located near salt and subsalt oil wells that had been impacted by losses due to extremely tight drilling margins (less than 0.5 lbs/gal) and geomechanical subsurface changes caused by severe reservoir depletion. A second group included oil-producing and water-injector wells prone to losses in excess of 300 bbl/hr due to induced fracturing while intersecting highly depleted reservoirs.

The initial approach, maintaining LPM concentrations by adding the material through sweeps, spotting pills on bottom and bypassing shakers, was somewhat successful in reducing total mud losses and loss per foot compared to conventional methodologies, but significant operational problems remained. This was likely the result of the LPM never reaching a high enough concentration to enable effective borehole protection by the FPR mechanism.

In one well, drilled as a sidetrack to an existing producing well, more than 4,000 bbl were lost while drilling, and 3,500 bbl of SBM were lost while running liner and cementing. A second well, drilled using FPR with MPSRS, experienced significant reduction in losses, and the drilling margin was improved enough to bring the well down uneventfully.

Another offset well saw losses of more than 35,000 bbl of SBM. The well was lost, and a sidetrack failed to reach its target. A subsequent well using the FPR method and an improved version of the MPSRS unit showed an 80% decrease in total losses with no barrels of formation losses.

In both cases the integrated workflow using FPR with MPSRS for WBS helped the operator significantly reduce NPT to reach targets that otherwise would have been unattainable. The objective going forward is to optimize the WBS process by further lowering LPM concentrations to reduce loading but achieve the same result.

As production companies continually look for ways to improve field economics through reduced NPT and other costs in ever-challenging plays, WBS solutions that are preventive rather than reactive are essential in drilling through difficult zones to reach prolific fields. A collaborative, integrated workflow that brings together a comprehensive suite of borehole strengthening tools, technologies, services and expertise, from planning to implementation to monitoring, is expanding the opportunities to drill the undrillable.