Mud Programme

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Mud Programme

Text of Mud Programme

  • Well PlanningMud ProgramA comprehensive drilling program should always be included as part of the preparations for drilling a well. It is beyond the scope of this manual to include details of mud engineering and mud chemistry, however, some of the options which have to be decided upon at the well planning stage will be discussed. Once the drilling engineer has formulated the general parameters for the mud system he needs, specialist service companies are often used to define the detailed specifications and material consumption for each interval.

    Well Planning 2004

  • Well PlanningMud ProgramThe engineer of course should always bear in mind the basicrequirements of mud system which are to:1.Control formation pressures.2.Carry cuttings to the surface.3.Clean the bottom of the hole.4.Cool and lubricate the bit.5.Transmit hydraulic power to the bit.6.Minimise formation damage.7.Minimise torque, drag and pipe sticking.

    Well Planning 2004

  • Well PlanningMud Program One system might not be able to perform these requirements throughout the course of drilling the well so it is quite possible that several different systems are required. These might also, on occasions, be conflict between the different requirements. It is then up to the drilling engineer to use his experience and knowledge to decide where the priority lies. The steps in preparing a mud program can generally be considered as follows:

    Well Planning 2004

  • Well Planning1.Establish the mud weight range required for each section of the hole with reference to the pore pressure and fracture gradient profiles and casing program.2.Study the geological prognosis for potentially difficult formations.3.Check the equivalent circulating densities, verify that the hydraulics are acceptable and determine a range of rheological parameters for each section.4.Establish the maximum fluid loss for each interval and determine the other properties such as pH solids content.5.Further considerations should enable optimum values to be assigned to the other parameters for the particular system to be used such as chlorides, hardness, salinity, clay content, or emulsionstability.

    Well Planning 2004

  • Well Planning6.Review and refine the program.

    7.Determine the requirements as regards surface equipment for treating and handling the mud.

    8.Develop contingency plans, if thought necessary for potential hole problems such as lost circulation or stuck pipe.

    9.Estimate the material requirements per interval.

    Well Planning 2004

  • Well PlanningMud Program

    When good offset well information is available this planning process is made much easier and the emphasis then should be on improving previous programs. Careful study of all the relevant data should at least result in the reduction of the mud system costs and could result in a significant reduction in drilling time.

    Well Planning 2004

  • Well PlanningWater Based MudsThese are the most frequently used types of mud, often being based on fresh water with the addition of weight material, clay or polymers and various other chemicals to produce the desired properties. When drilling through some shale sections however, the use of fresh water can cause the shale to swell and hole problems due to sloughing can follow. When this is anticipated an inhibited system can be built up with the addition of gypsum, lime or potassium chloride. Salt water based muds, nearly always used offshore, could also be classed as inhibited muds. If large sections of salt are to be drilled then in order to eliminate the development of large cavities, the mud system would need totally inhibited by initially saturating it with salt.

    Well Planning 2004

  • Well PlanningWater Based MudsDispersed systems are often run especially when drilling through high activity clays. These systems include thinners or dispersants to make the clay platelets separate in the water phase and providebetter viscosity and filtration control. A gel-lignosulphonate is a commonly used dispersed system. Typical viscosifiers for water based systems are bentonite and polymers such as HEC and CMC.

    Well Planning 2004

  • Well PlanningWater Based MudsTo great a viscosity caused by excessive colloids, undesirable drill solids or contaminants such as cement can result in gel strengths and yield points that are too high. This can be treated by the addition of more water (which in turn might require the use of more barite and other chemicals) or by adding thinners to the system if the other mud properties are not restrictive.

    Well Planning 2004

  • Well PlanningWater Based MudsAll muds will lose fluid to a permeable formation and create a filter cake. To reduce the degree of invasion of the formation by mud filtrate, fluid loss agents are added to the mud to help make thefilter cake thin and only semi-permeable. Due to the chemical make-up of drilling fluids and the nature in which they react the pH or a water-based fluid should be maintained in the alkaline range. This is usually done with the addition of caustic soda to compensate for any acidic chemicals or contaminated water.

    Well Planning 2004

  • Well PlanningWater Based Muds

    High density water based systems often give rise to solids control problems. Extra solids will originate from the drilled formation, weight material, gels and other chemicals. Simply diluting the system with more water in order to restore the balance can be a very costly solution since it often results in additional chemicals being consumed and a wasteful volume of mud being built. The proper solution to employ for when such problems are anticipated is to plan for the use of good solids control equipment and to plan in advance for suitable chemical additions to reduce the hydration of formation clays and provide filtration control.

    Well Planning 2004

  • Well PlanningOil Based MudsMost modem oil base mud systems use a mineral oil phase that is more environmentally acceptable than previously used diesel oil phases. These systems have been the cause of some very significant reductions of hole problems and improved drilling performances when used in sections containing hydratable formations. In such circumstances the water phase of the system (usually from 5% to 30%) needs to have a salinity level greater than that of the formation. This is controlled by additions of calcium chloride.

    Well Planning 2004

  • Well PlanningOil Based MudsEmulsifiers are used to form an oil film completely round the water droplets. Their performance can be improved by ensuring the mud is well "sheared" usually with mud guns. The water phase helps tosupport barite, contributes to gel strengths and viscosity, acts as a fluid loss agent and allows many of the other chemicals additives to dissolve.The stability of the oil/water emulsion is measured by recording the mud's electrical stability though if contaminants such as magnesium chloride are present then this test can be invalid. In this case the HP/HT fluid test can be performed and the filtrate checked for traces of water.

    Well Planning 2004

  • Well PlanningOil Based MudsSpecial additives to act as emulsifiers, viscosifiers, dispersants and fluid-loss agents are available for use with oil based systems. Products are also provided for formulating cement spacers that promote the water wetting of oil wet surfaces. The viscosity of clean oil muds is highly temperature sensitive and requires careful attention. It is also usual to run excess lime as a contingency against acidic gasses such as hydrogen sulphide or carbon dioxide. As stated modern oil based systems, although expensive to build and maintain can prove very cost efficient in the right circumstances. They should always be considered for use, logistics and legislation permitting in potentially troublesome or deviated holes.

    Well Planning 2004

  • Well PlanningOil Based MudsOne inconvenience is their readiness to completely absorb gas in solution at even fairly low pressure and then release it suddenly when the pressure is reduced. This can make a gas influx into thewellbore very difficult to detect until it is circulated up nearly to surface. Its presence then can often be felt very drastically. The oil's ability to retain its temperature and cause ventilation problems in enclosed shaker and pit rooms is another inconvenience as might be the need to modify some of the rig's fluid handling and treatment equipment if it is not already designed for working with oil based systems.

    Well Planning 2004

  • Well PlanningBrinesBrines are used extensively in completion and workover operations where a low solids content is important. They are used occasionally in exploration or appraisal wells for the testing phase for similar reasons. In order to be effective they should be filtered prior to use.

    Well Planning 2004

  • Well PlanningTreatment Equipment

    Once the mud system has been planned it remains to make sure that the rig has suitable handling and treating equipment available. It would make little sense for example to design an expensive oilbased system and then find that its weight could only be controlled by dilution due to the rigs inadequate solids control equipment.

    Well Planning 2004

  • Well PlanningTreatment Equipment

    Most rigs have a standard circulating system however some tend to be more flexible and hence better than others. This is particularly true of offshore units. The problem concerning maintaining and conditioning mud properties always revolves around the solids control equipment. It should be designed to remove very small particles down as small as one micron. The following are some of the solids control equipment essential to good ope