FORMULATION OF PROBLEM
Lost circulation is a common drilling problem especially in highly permeable formations, depleted reservoirs, and fractured or cavernous formations. The range of lost circulation problems begin in the shallow, unconsolidated formations and extend into the well-consolidated formations that are fractured by the hydrostatic head imposed by the drilling mud (Moore, 1986). It can then be defined as the reduced or total absence of fluid flow up the formation-casing or casing-tubing annulus when fluid is pumped down the drill pipe or casing. The industry spends millions of dollars every year to combat lost circulation and its associated detrimental effects such as loss of rig time, stuck pipe, blow-outs, and less frequently, the abandonment of expensive wells. Two conditions are both necessary for lost circulation to occur down hole: 1) the pressure in the well bore must exceed the pore pressure and 2) there must be a flow pathway for the losses to occur (Osisanya, 2011). Sub-surface pathways that cause, or lead to, lost circulation can be broadly classified as follows:
Induced or created fractures (fast tripping or underground blow-outs) Cavernous formations (crevices and channels)
Unconsolidated or highly permeable formations
Natural fractures present in the rock formations (including non-sealing faults)
The rate of losses is indicative of the lost pathways and can also give the treatment method to be used to combat the losses. The severity of lost circulation can be grouped into the following categories (Abbas et al. 2004):
Seepage losses: up to 10 bbl/hr lost while circulating
Partial losses: 10 – 500 bbl/hr lost while circulating
Severe losses: more than 500 bbl/hr lost while circulating
Total losses: no fluid comes out of the annulus
Circulation may be lost even when fluid densities are within the customary safety-margin; less dense than the fracture density of the formation. Stopping circulation losses before they get out of control is crucial for safe and economically rewarding operations (Abbas et al. 2004). According to Ivan and Bruton (2003), “Deepwater drilling has brought loss circulation control to a more critical level as it involves narrow pore-pressure/fracture-gradient windows, cold drilling fluid temperatures, high equivalent circulating densities (ECDs), high cost-per-barrel of synthetic-based fluids (SBM) and a high cost for rig time/non-productive time (NPT).” The reduction of the fracture pressure gradient in the deeper water is mainly due to the low stress regime as a result of the reduction in the overburden pressure gradient. Also, drilling through sub-salt zones poses a challenge to the operator because of the problem of lost circulation encountered in these zones. These wells have shear zones above and below the salt formations and also narrow margins between the pore and fracture pressure and hence these wells tend to register severe losses in circulation.
1.2 LITERATURE REVIEW
Lost circulation is a broad subject and several studies and measures have been introduced in the industry to combat it. For example, Moore, (1986) noted that in shallow, unconsolidated formations where the drilling fluid may flow easily into the formation, the most common method used to combat lost circulation is to thicken the mud. This may be done in fresh water muds by adding flocculating agents such as lime or cement. He also stated that in areas such as below surface casing in normal-pressure formations where natural fractures are common, the most common method used to combat lost circulation is to drill without fluid returns to the surface. The purpose is to remove the generated cuttings from the hole and deposit them at the lost circulation zone. However, this practice requires large volumes of water and close supervision as there is the possibility of encountering high drill-string torque and drag.
Current research on lost circulation has been focused on the use of Lost Circulation Materials (LCMs), especially chemical formulations which have been proven to be more effective. Hamburger et al. (1983) of Exxon Production Research Company developed a Shear-thickening Fluid (STF) which was tested successfully in 10 different wells that experienced severe lost circulation. A STF is a multi-component system composed of water-swellable material (usually clay) dispersed in an oil-external emulsion. The emulsion consists of liquid oil, an oil-soluble surfactant, and aqueous-phase droplets containing dissolved polymer. At the low shear rates encountered while it is being pumped down the drill pipe, the fluid is a low-viscosity, pumpable liquid. Yet as it passes through the drill-bit nozzles, the resulting high shear rates cause the fluid to thicken irreversibly into a high strength viscous paste.