July 18, 2019
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Rheological properties that control the effectiveness of a drilling fluid with regard to hole cleaning and explain how they influence cuttings transportation. Identify all the options available to manage the annular pressure during drilling operations.
First drilling fluids can be made of water based solution, polymer solution or an oil based solution. The hardest fluid to maintain optimal drilling fluid properties is the water based solution, yet the cheapest. The water based solution can be dehydrated and loose it’s effective properties quickly due to downhole friction, downhole temperatures, and downhole pressures or simply the formations it is being used in. The rheology of a fluid can change quickly from solids build up in the solution or from temperature and pressure, seepage is also a problem as some formations soak up the water from the fluid solution downhole such as clay. If the plasticity properties of the fluid get too high they loose their effectiveness in carrying cuttings to surface or also if they are too low. This also largely depends on many factors such as flow rate, salt content in the water being used that impact shear stress of the fluid.
In order to manage pressure while drilling the dynamic annular pressure is the static annular pressure plus pressure drop along the annulus of the well bore. This pressure drop is created by the friction force between the mud particles and the drilling string to allow mud flowing up the annulus. So, BHA design is a critical factor in managing drilling operations and go hand in hand with fluid design.
Viscosity, shear rate, flow rate and solids content dictate the annular pressure and of course mud weight. Other methods to determine the annular pressure while drilling include sensors in the BHA such as pressure and temperature. The annular pressure at surface and the mud weight in compared to the mud weight out as well as careful monitoring of the cuttings as they go over the shale shakers. APWD has long since developed and has become more highly sophisticated compared to -10 years ago. It is still not a matter of getting a reading from a BHA as those readings can be limited and only provide one point in the drilling string depending.
Over the last 20 years there has been some significant developments in managing pressure while drilling through downhole sensors to determine the ECD to ensure hole cleaning is at its optimal as hole cleaning can cause a drilling problems. High ECD’s can also cause problems and depends on the formations being drilled through. All of hydraulics in drilling play an important factor on the hole sizes required to drill a well, the number of casing strings needed to isolate different pressure formations because today wells are being drilled to greater depths to find the reserves and produce oil. There is not one all fit solution for every well however the more information that can be obtained downhole the better prepared the engineer is for the next section to be drilled. Determining pore pressures through seismic is also key and critical technology so as to ensure optimal hydraulic design. High ECD’s can help but can lead to other problems, if you are to maintain a high ECD the mud weight must be low but must still be above pore pressure. If an ECD is too high it can fracture the formation leading to fluid losses and therefore the APWD system is part of this process. By keeping annular pressures high you can drill with lower mud weights, however once the circulation system stops problems can occur and therefore during APWD the circulation needs to be maintained.
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