Where severe drilling conditions exist, especially the combination of water-sensitive sloughing shales, abnormal formation pressures and high bottom-hole temperatures, several operators in South East Asia regularly use "invert emulsion" oil muds to drill and complete trouble-free gauge holes.

This paper will review oil mud operations in the South-East Asian region, and a case history will be presented to illustrate the successful use of oil mud in the development of a major Indonesian gas field.

Some misconceptions still surround the use of oil muds. Modern oil muds are stable, practical and relatively simple drilling fluids which may be scientifically formulated to match the most severe drilling conditions. This paper will attempt to clarify some of the misunderstandings commonly associated with oil muds.

The range of application and advantages of an oil mud will be demonstrated, and the physical and chemical properties of a typical invert emulsion oil mud will be discussed.

Finally, the key to an efficient oil mud program stage. Using examples from one Indonesian operator's extensive experience, the various economic and logistic factors to be considered when using oil mud will be discussed.

Oil Mud Country

South-East Asia is one of the world's more difficult drilling areas. High geothermal gradients occur throughout the region and geopressured, water-sensitive shales are often present. Drilling problems typically include swelling shales, tight hole, bridging and problems typically include swelling shales, tight hole, bridging and stuck pipe. High mud densities are often required, and several areas posses hydrocarbon reservoirs containing water-sensitive clays. posses hydrocarbon reservoirs containing water-sensitive clays. Through inhibitive salt-polymer mud systems are becoming more popular, the only fluid capable of overcoming all these drilling popular, the only fluid capable of overcoming all these drilling problems is oil mud. problems is oil mud. In most cases, operators in South-east Asia use oil mud for two reasons:

  • A well cannot be drilled to total depth with a water base mud.

  • To reduce overall well costs.

In Indonesia, oil mud is used regularly to drill geopressured, water-sensitive shales in North Sumatra and Kutei Basins, and has also been used for shales control in the East Java (Mudura), North West Jave (Sunda). Central Sumatra and West Natuna Basins (Figure 1). The Kutei Basin also contains deep shaly-sand reservoirs which are susceptible to damage by filtrate invasion from water base drilling and completion fluids.

One well in the Sunda Straits between Java and Sumatra used oil mud to drill underbalanced through geopressured shales with interbedded fracture zones of volcanic ash.

Oil mud has also been successfully used to control sloughing shale in the Malay Basin of West Malaysia, the Sarawak Basin of Brunei and East Malaysia and The East Palawan Basin of the Philippines.

Oil mud packer fluids have also been used successfully on production wells in Indonesia. Brunei and Malaysia.

Advantages Of Oil Mud

Oil muds have many advantages over water base drilling and completion fluids, and may be used to:

  • Prevent damage to water-sensitive reservoirs

  • Control water-sensitive shales

  • Resist high bottom-hole temperatures

  • Drill gauge hole

  • Control salt sections

  • Allow underbalanced drilling in shale

  • Extend bit life

  • Improve drill string lubrication, i.e. reduce rotating torque and minimize wall-sticking tendencies

  • Eliminate corrosion of drillstring and tubulars

These benefits are over and above the regular functions of a water base fluid, e.g. cuttings removal, control of formation pressure and bit cooling.

Shale Control

The most important feature of oil mud is its ability to prevent swelling of water-sensitive clays and shales.

All water base muds water as the primary or "continuous" phase, so no matter how high the mud salinity or how low the phase, so no matter how high the mud salinity or how low the filtrate loss, some water will always come into contact with the borehole wall when water base muds are used. If the formation exposed to the well bore contains clay or shale, then these will tend to absorb water into the clay structure to compensate for the water squeezed out during compaction. This effect is termed surface hydration.

The force exerted by the shale to re-absorb the water lost during compaction is termed the surface hydration force. This force occurs when a borehole is drilled into a shale formation and the confining stress due to the weight of the overburden is relieved. This surface hydration force exists for all shales below a few thousand feet and its magnitude is approximately equal to the "vertical matrix stress" of the formation. Using data from the U.S. Gulf Coast, the matrix stress is found to have an approximate value of 0.535 psi/foot in normally compacted shales, and value somewhat less than this in geopressured shales. Table 1 lists shale surface hydration force versus depth for normally compacted shales.

All shales exhibit this force, and all water base muds will give some of their water to the shales. Encapsulating polymers may be added to a water base mud to provide a polymer film over shales exposed to the borehole.

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