Borehole Fluid Mechanics and Mud Design

It is important to achieve the correct hole cleaning, to reduce friction and to minimise formation damage (permeability loss around the borehole). It is equally important to avoid fracturing the strata being drilled by excessive pressure. In some instances, such as in directionally deviated or horizontal holes, it is vital to maintain a turbulent flow regime so that cuttings are properly cleared from the hole. In many vertical holes the opposite is the case, and a laminar regime is needed that uses a strongly shear thinning mud which will form a near gel to prevent the fall back of cuttings during pipe changes.

Sigra provides such an independent service to achieve optimal drill hole hydraulics. The key components of this are:

Mud Additives

A knowledge of the behaviour of the various mud additives – bentonites, polymers, xanthan gum etc.

Rheological Testing

Sigra has a large size rotational viscosimeter which behaves in a hydraulically similar manner to the typical small units. However it has the advantage that it can handle muds or cement grouts containing particulate matter because the gap between the rotating sleeve and the torque measuring bob is greater. Testing can be conducted at a range of temperatures to suit borehole conditions. The results are reported in terms of viscosity versus shear rate and the traditional 300 and 600 rpm viscosimeter readings as well as appropriate constants for Bingham or power law models.

Frictional Testing

Sigra can determine the coefficient of friction between any drill pipe material and a rock sample at controlled displacement rate.

Drilling Mud Modelling

Sigra has a computer simulator that models the absolute pressure in a borehole along its length due to fluid flow. It also calculates the pump pressure required to deliver the flow. The basis of the model relies on the hole and drill pipe geometry and the rheological behaviour of the drilling fluid. The model will handle fluids that behave according to Newtonian, Bingham or power law rheological characteristics.