Lost circulation is one of the biggest contributors to drilling non-productive time, and it is the most difficult segment of drilling in which to make economic decisions. Estimations of economic impact in this segment vary widely, but it is safe to say that it represents a very large portion of the total non-productive expense for drilling a well. As rig rates increase, the economic impact of non-productive time increases as well. Therefore, any technology that reduces drilling non-productive time can translate into millions of dollars in reduced drilling costs.
Conventional fracturing theory predicts that lost circulation may occur when the tangential stress at the borehole surface exceeds the tensile strength of a rock. However, this conventional theory could not explain why lost circulation occurs more frequently when oil-based drilling fluids are used.
The most significant result was not that lost circulation could be controlled by these treatments, but that the resistance to lost circulation (increased wellbore strength) could be enhanced significantly.3
Figure 1: Improved Wellbore Pressure Containment (Click to enlarge)
Halliburton has combined these concepts and principles into its WellSET treatment. The WellSET treatment is a wellbore strength enhancing technology that increases the "hoop stress" (and thus the wellbore pressure containment ability) in the near wellbore region. This is achieved by placing a plugging material in an induced fracture to prevent further pressure and fluid transmission to the fracture tip, while at the same time widening and propping the fracture (Fig. 2).
Figure 2: Increased Hoop Stress (Click to enlarge)
Figure 3: WellSET Treating Materials (Click to enlarge)
Chemical lost circulation treatments that form a deformable, viscous and cohesive material also may have the ability to improve wellbore pressure containment as long as they can increase compressive stress at the fracture face. They include materials such as FUSE-IT™ LCM, FlexPlug® OBM and W sealants, and FDP C-735 sealant (Figure 4).
Figure 4: FUSE-IT Chemical Sealant (Click to enlarge)
The chemical sealant systems are designed to react with the drilling fluid to create highly viscous and cohesive sealants in the wellbore that are displaced into the lost circulation fractures. FUSE-IT lost circulation treatment reacts over a wide range of water-based fluid mixing ratios; FlexPlug OBM slurry reacts with oil-based fluids. The FUSE-IT treatment is also capable of reacting within an oil-based fluid if a separate water pill is provided. These drilling fluid-reactive systems are not dependent on temperature or pressure, thus removing a significant amount of placement uncertainty present with competitive systems.
Halliburton DFG™ software with DrillAhead® hydraulics module is used to predict the ECD over an interval (in one module), calculate the width of a fracture that may be initiated, and select and design a proper material and particle size distribution that can efficiently prop and plug that fracture (in a second module) (Figure 5).
Figure 5: DFG Near Wellbore Fracture Module (Click to enlarge)
A third module then predicts the change in rheology resulting from the addition of the specialized lost circulation materials, which is cycled back to update the ECD calculations (Figure 6).
Figure 6: Predicted Rheology after LCM Addition (Click to enlarge)
Contingency chemical sealant treatment applications are designed using the DrillAhead® platform software package that models long fractures and provides fracture characterization for them. The software package also models lost circulation fracture length and width vs. ECD and provides planning for a chemical sealant treatment for lost circulation mitigation
An operator working in the East Cameron area of the Gulf of Mexico wanted to sidetrack an existing well to increase production rates by cutting a window in the existing 9 5/8-in. casing and drilling to a total depth of 11,145 ft MD/ 10,515 ft TVD using oil-based fluid (OBF). However, there was significant risk of lost circulation in three severely depleted sand sections.
After the operator provided the data relative to the sand sections to the Halliburton team, DFG™ software was used to design the WellSET™ treatment for pre-treating the entire system. This determination was based on the anticipated permeability and subsequent pore sizes of the depleted formations.
The treated system was also supplemented with 15-20 bbl sweeps which were pumped every 100 ft. Each depleted sand section required a different treatment regimen according to the results determined by the WellSET treatment design.
The first depleted sand was drilled with a 10.0 ppg fluid. The sand package was depleted to a 5.7 ppg pore pressure. The equivalent differential pressure was +/- 1,095 psi. This section was treated with:
The operator was successful in drilling each depleted sand package with no losses to the formation. Halliburton's Wellbore Stress Management™ service utilizing the DFG software virtually eliminated the "guess work" in designing the proper WellSET lost circulation treatment for the sand packages.
1Morita, N., et al., "Theory of Lost Circulation Pressure," SPE 20409 SPE Annual Technical Conference and Exhibition, 23-26 September, New Orleans, Louisiana (1990).
2Fuh, G. F., et al., "A New Approach to Preventing Lost Circulation," SPE 24599 SPE Annual Technical Conference and Exhibition, 4-7 October, Washington, D.C (1992).
3Fuh, G. F. et al. "Method for Inhibiting the Initiation and Propagation of Formation Fractures while Drilling and Casing a Well", US Patent 5,207,282 (1993).
4Alberty and McLean, "Fracture Gradients in Depleted Reservoirs - Drilling Wells in Late Reservoir Life", SPE/IADC 67740 SPE/IADC Drilling Conference, 27 Feb-1 Mar, Amsterdam, The Netherlands (2001).
5Sweatman, R. et. al., "Formation Pressure Integrity Treatments Optimize Drilling and Completion of HTHP Production Hole Sections," SPE68946, SPE European Formation Damage Conference, 21-22 May, The Hague, Netherlands (2001).
6Aston, M, et al., "Drilling Fluid for Wellbore Strengthening," SPE/IADC 87130 IADC/SPE Drilling Conference, 2-4 March, Dallas, Texas (2004).
7Alberty, M. and McLean, M., "A Physical Model for Stress Cages," SPE 90493 SPE Annual Technical Conference and Exhibition, 26-29 September, Houston, Texas (2004).
8DuPriest, F., "Fracture Closure Stress (FCS) and Lost Returns Practices," SPE/IADC 92192 SPE/IADC Drilling Conference, 23-25 February, Amsterdam, Netherlands (2005).