The concept of water-in-oil (invert) drilling fluid technology provides fluid systems that are economical, fast drilling, and non-damaging to either the reservoir or the environment. The PetroDrill system is specifically formulated to satisfy these requirements. The highly concentrated chemical components allow formulation for a variety of temperature and pressure conditions.
The oil external phase and thin wall cake virtually eliminate differential sticking while providing the lowest possible torque and drag values. The base oil and amine emulsifiers combine to provide corrosion protection for the drill string and casing in the presence of corrosive or saline environments. The PetroDrill system can be formulated to be stable to temperatures in excess of 450°F and pressures greater than 20,000 psi. The internal phase is typically formulated with Calcium Chloride but can also be formulated with Sodium Chloride. These salts, in a wide range of concentrations, provide a flexible activity range to prevent shale hydration or hole enlargement. Calcium Chloride is used in preference over Sodium Chloride because of the greater water phase salinity, higher activity, and higher density. Drilling performance is enhanced by the water free filtrate which does not allow formation clays to swell. “Wiper trips” (also called “short” or “drag” trips) which are necessary when using water base mud to drill through high clay content (swelling formations) can be eliminated. This can produce significant savings in rig time and cost.
The external phase of a water base fluid or an oil-in-water emulsion fluid is water. The water comes in contact with the well bore and producing formations. Materials added, such as Barite, become water wet. Clays yield by hydration with water, both when added as a viscosifier and when the mud or filtrate contact clays in the drilled formations.
An invert emulsion has oil as the external phase and water as the internal phase. Objects which come in contact with the mud become oil wet. The oil comes in contact with the well bore and producing formations. Clays do not yield unless specially treated to be organophillic (oil loving).
A continuous external phase of oil must be maintained to prevent water wetting of solids. This is particularly true in weighted muds. Water wetting of Barite in the system can cause catastrophic settling of solids which cannot be stirred back into suspension. Therefore, the oil to water ratio must be maintained at greater than 55/45 in all cases.
The oil used in the external phase will govern many of the properties by which the drilling fluid will be evaluated. The component chemicals used to formulate PetroDrill systems are compatible with a broad range of oils. The diverse properties of the various oils provide exceptional flexibility with respect to flow properties, gel structure, toxicity, temperature stability, filtration values and cost effectiveness.
LVT 200, a very low aromatic, “low toxicity”, low viscosity oil is available for use in oil base drilling fluids. It is from this oil that most PetroDrill systems are prepared. When properly applied, the PetroDrill systems are suitable for use offshore and in sensitive land base areas. Cost and availability may dictate the use of diesel or other oils as the external phase. Emulsifiers specific to the base oil properties are used.
The internal phase of PetroDrill is salt water. The water phase controls, to a great extent, the rheological and filtration properties of the fluid. By varying the oil/water ratio the viscosity and rheological properties of the mud can be controlled. By varying the salinity of the water phase, the “activity” of the fluid may be altered with respect to shale hydration and well bore stability.
In most drilling operations a “balanced” activity is recommended, so that water vapors do not transfer from the mud to the well bore or vice versa. Except in critical conditions, a simple daily check of the hardness of the cuttings at the shaker is sufficient. In highly deviated holes or troublesome shales, a hygrometer should be used to monitor the “activity” of the fluid and cuttings. The hygrometer measures the relative humidity produced by the fluid and cuttings. Fresh water has an “activity” of 1.0, saturated Sodium Chloride Brine 0.75, and saturated Calcium Chloride Brine 0.30. An activity of 0.60 will satisfy most shales encountered.
A saturated CaCl2 brine is not recommended as it is just that, saturated. It will be difficult to solubilize the various additives that must go into the water phase.
Once the formation activity has been balanced by the mud, further increases in salinity can only make the fluid more costly to build and expensive to maintain. Therefore, increases above this balance are not recommended.
Further, where possible it is recommended that the salt additions be made with brine fluids at either a pre-adjusted chloride level or at saturation. For NaCl, only brine water can be used. Crystalline Sodium Chloride (sack) can be added to a finished emulsion. However, the NaCl will drop out in the continuous oil phase without penetrating the oil water interface if high levels of Calcium Chloride are present or the NaCl is near saturation. Powdered Calcium Chloride can be added if necessary. Do not mix NaCl and CaCl2.
ADVANTAGES OF USING THE PetroDrill SYSTEM
Protection of Producing Sands
A properly maintained invert emulsion mud will produce an all oil filtrate. This effectively prevents the swelling of formation clays which can reduce permeability within the sand.
Protection of Soluble Formations
The oil external phase permits soluble formations such as Salt, Potash, and Gypsum to be drilled without detrimental effects to the formations or the mud system.
Prevention of Stuck Pipe
The lubricity of PetroDrill produces an extremely low coefficient of friction. The ultra-thin all-oil filtrate reduces the contact area in potential differential sticking areas. Clay swelling is virtually eliminated avoiding one of the major causes of tight hole, the need for frequent trips, and a contributing factor in stuck pipe situations.
High Temperature Application
The PetroDrill system is not adversely affected by temperature as the emulsion is stable in excess of 450°F. In HTHP applications the stable rheology and gel strengths allow operations to proceed at minimum values between fracture gradient and pore pressure.
Torque and Drag Reduction
The excellent lubricity coefficient, thin filter cake, and inert nature of the PetroDrill system will reduce torque and drag in deep and deviated holes. Directional sets and slides can be accomplished in minimum times.
The oil external phase and amine derivative emulsifiers combine to provide maximum drill pipe and casing protection from such corrosives as oxygen, hydrogen sulfide, chloride salts, carbon dioxide, and organic acids. The PetroDrill systems are excellent packer fluids because they are not susceptible to temperature degradation nor are they miscible with corrosive formation waters.
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