MUD TANK AND PIT USE

The surface mud system consists of the flow line, active mud tanks, reserve mud tanks, trip mud tank, mud agitators, pumps, motors, solids- and gas-removal devices, mixing and shearing devices, and associated piping. The surface mud system can be considered to be composed of the following sections. Each section has unique agitation and suspension requirements. Please refer to Table 10.1 for TOR requirements for the compartments. Remember that TOR is an indication of how vigorously the fluid must move within the compartment.

Removal

All compartments of a system that use solids-control equipment or degassing equipment require proper agitation, except the sand trap, if the sand trap is used as such. Solids-control equipment works best when solids loading remains constant. Slugs of a large quantity of solids tend to plug hydrocyclones and centrifuges. This leads to downtime and decreased solids-removal efficiency, with the resulting consequences.

Addition

The equipment and tanks utilized in the addition and blending of mud additives require proper agitation. As stated in the early part of this chapter, the purpose of the surface mud system is to allow maintenance of the mud before it is pumped back down the hole. Part of the reconditioning process involves the addition of mud materials and chemicals to the mud system. The addition of materials is also required when the system volume is increased or the mud properties (such as weight or viscosity) are changed. In critical situations, such as well-control problems, it is desirable to be able to mix the mud materials rapidly and thoroughly. The purpose of any additional equipment is to mix mud materials and chemicals into a fluid with minimum balling, maximum speed, and safety to personnel.

A properly designed mud system will have adequate storage and
mixing capacity. For existing systems whose capacity is taxed, auxiliary premix systems should be used; especially when rapid shearing is required. Premix systems are useful for blending bentonite or hard-to mix polymers, such as CMCs (ceramic matrix compounds), PHPA (partially hydrolyzed polyacrylamide), XC (xanthan gum, from the
bacterial genus Xanthomonas campestris), and many other fluid additives. They also provide extra capacity, which is especially useful if the mud type is to be modified and requires isolation of one mud type from another (e.g., when changing from an aqueous fluid to a nonaqueous fluid).

Suction

All compartments in this section require proper agitation. This section contains the tank(s) and/or compartment(s) from which the rig pumps take suction, including any associated pumps (such as charging pumps) used to deliver fluid to the well or trip tank. Usually included is a pill/ slug compartment. The pill/slug tank is used to prepare a drilling fluid with a higher density or extra hole-sweeping ability than usual; generally, a 20- to 50-bbl volume is sufficient. The denser nature of the fluid requires that pill/slug compartments have more vigorous agitation than any other compartment. An agitator and mud gun combination are ideal for these compartments. Mud gun suction should come from the slug tank. Thus, recirculation within the compartment promotes maximum homogeneity and precludes dilution of the pill or slug with fluids from other compartments.

Reserve

The tank(s) or pit(s) and associated equipment used to isolate mud from the active system all require proper agitation. Compartments predominantly designated for long-term storage of drilling fluid (e.g., large-volume holding tanks) will not need strong shear forces or high TOR. Contour-type impellers have the added benefit of requiring less horsepower per unit of fluid displaced, so they are ideally suited for this
purpose.

Discharge

The tank(s) or pit(s) and equipment located at the well site used to store and process mud and cuttings for disposal form the discharge function. Special consideration for discharge tanks may be required. Due to the variety of solid and fluid combinations encountered with discharges, it is outside the scope of this discussion to make recommendations. Please contact a qualified service provider to determine agitation requirements.

Trip Tank

This mud tank, and associated equipment, is used to isolate mud from the active systems for gauging pipe displacement during tripping operations. No agitation is required under normal conditions.