Deepwater drilling has brought the problems and associated costs of lost circulation (LC) to new levels, but an innovative software program lets users explore and assess operator and well-specific LC problems and link them to available solutions.

The problem of lost circulation (LC) became apparent in the early history of the drilling industry and was magnified considerably when operators began drilling deeper wells and through depleted formations. The industry spends millions of dollars a year to combat LC and the detrimental effects it propagates, such as loss of rig time, stuck pipe, side-tracks, blowouts and, occasionally, the abandonment of expensive wells. It is estimated that LC costs the industry about US $800 million per year, while the LC products could represent as much as $200 million.

Deepwater drilling has brought LC control to a more critical level as it involves narrow pore-pressure/fracture gradient windows, cold drilling fluid temperatures, high equivalent circulating densities (ECD), high cost-per-barrel synthetic-based fluids (SBM) and high cost for lost rig time/non-productive time (NPT).

The process

Traditionally, LC control has been a reactionary process. The lost circulation assessment and planning (LCAP) process involves proactively exploring and assessing project-specific LC problems and linking them to existing LC products, systems and services.

The process and software uses data from existing resources - offset well analysis, logs, mineralogy/geology analysis - as inputs, and focuses on available products such as conventional lost circulation material (LCM), specialized polymer crosslinked pills, software tools and to create project-specific plans and solutions. For maximum success, emphasis is placed on assessment and planning rather than individual products. This methodology prevents and minimizes LC problems instead of pursuing an after-the-fact cure. Careful preplanning simplifies the process of choosing from the 177 specialized LC products offered by 46 different drilling-fluid suppliers. Figure 1 shows the five key elements of the process.

During the exploration phase, candidate wells for the LCAP process are identified, the magnitude of the LC potential is assessed and strategic plans developed. During development, a technical development team analyzes the offset wells' data to identify the most probable thief zones, links existing LC products and technology with potential for solving the specific problem and uses existing engineering tools to develop the LCAP program.

The LCAP software is an important tool used in the development phase, which includes these three steps:

Step 1. Gather and process all the available project-related and problem-specific data. This represents a key difference from the after-the-fact, reactive approach to LC. The type of data available includes:

• Pattern of LC as part of geology, lithology and stratigraphy analysis;

• Pore pressure and fracture gradient;

• Logging data (imaging, wireline, pressure-while-drilling and array resistivity logs);

• Drilling reports that indicate pre-loss and post-loss drilling conditions, and drilling events, to name a few;

• Offset well LC analysis, such as treatments and results, and lessons learned;

• Hydraulics analysis, pressure loss and ECD simulations; and

• Evaluation of historical cost associated with LC.

Step 2. Identify the most probable thief zone (type and location). The type of thief zone refers to the nature of the loss zone, such as induced fractures, natural fractures and permeable formations, and the location refers to the relative position of the loss zone. The types of tools/methods available for thief zone identification are:

• Pressure transducer surveys, openhole logs, hot wire surveys, radioactive transducer surveys, temperature surveys and spinner surveys;

• Best practices and the pre-loss events analysis;

• Real-time geomechanical analysis methods; and

• If available, the use of LC expert software tools, which require an understanding and knowledge of pre-loss conditions.

Step 3. Identify the best conventional LC treatments and recommend subsequent contingency specialized treatments, such as crosslinking pills and gunk squeezes, including detailed operational procedures.

At the end of this step, a formal project-specific LCAP should be developed and provided to the well execution team for implementation and execution.

The recommended LC treatments should always include conventional LCM pills and treatments, such as blended pills of granular and flaky and fibrous materials, and specialized LC treatments, such as crosslinking technology and oil gellant pills, etc. Always include pre-treatment procedures (LCM mixed in the whole drilling fluid system). Check with the drilling/directional engineer and the measurement-while-drilling (MWD) and logging-while-drilling (LWD) operator for the smallest passage in the downhole tools and verify the particle size distribution of the chosen LCM passes through the tool openings. Include detailed mixing and spotting procedures for all the pills that will be mixed as a stand-alone spotting pill treatment. Make sure considerations are given to the rig-specific mixing equipment and mixing tanks.

Identify the pill mixing tanks/pits and determine the dead volume. Always consider the dead volume in the mixing procedures. For all crosslinking pills, best practices include the use of cementing units for pumping and squeezing.

Induced fractures in deep water

Planning for a deepwater subsalt Gulf of Mexico well included the LCAP process because of expected LC problems as experienced during the offset wells.

Three intervals were analyzed: 141⁄2-in., 121⁄4-in. and 97⁄8-in. Miocene-type lithology (shale and sandstone) typified the first two intervals; the bottom section was a transition Miocene-Oligocene with the possibility of limestone stringers. The thief zone immediately below the salt formation (the first interval) was expected to be a typical rubble zone of highly fractured shale.

The fracture gradient in the rubble zone was less relevant but could present important variations (much lower values) from the estimated range. A development team was assembled, all the necessary data was gathered, and an LCAP process document was generated. The highest potential loss zone to be encountered, other than the rubble zone, was determined to be induced-fractures shale, followed closely by porous sandstone, induced-fractures sandstone and naturally fractured sandstone.

Wells of this nature are typically drilled with SBM. Some types of material, such as synthetic graphite, have a neutral surface and can work in both types of fluid - water-based mud and SBM. Accordingly, a typical formulation for curing seepage losses may contain a blend of granular calcium carbonate and synthetic graphite. Optional treatments, ranging from conventional to specialized, follow:

• Pre-treating the whole mud system with calcium carbonate (8 lb/bbl to 10 lb/bbl) and synthetic graphite (8 lb/bbl to 10lb/bbl);

• Mixing fiber-based particulate LCM (20 lb/bbl to 25 lb/bbl) in the system as passing through the seepage loss zone;

• Pumping 15 lb/bbl to 20 lb/bbl synthetic graphite sweeps (25 bbl to 50 bbl volume) at a frequency of one sweep every third stand, and if needed, as often as one every stand; and

• Spotting a sequence of blended LCM pills as needed starting with 35 lb/bbl, then 65 lb/bbl and 95 lb/bbl, respectively, followed if needed by a soft set pill (such as PCP) or a reverse gunk squeeze.

A series of LC decision trees were developed to address LC problems for this deepwater prospect.

Conclusion

The LCAP process and software helps project teams specify project-, well- and interval-specific criteria to identify possible lost circulation problems and identify solutions before a problem arises. This time- and cost-saving strategy can be particularly advantageous for deepwater projects where the costs and the stakes are high.