Clayless Drilling Muds with Reduced Density and Low Fluid-loss Rates

Today drilling contractors often have to develop field areas that were previously regarded as being of little promise.

The requirements for the development of such reservoirs have to protect the reservoir's original poroperm properties against overbalance effects, and also against the impact of drilling fluid filtrates and solids. However, in the course of drilling-in operations on such wells, it is possible to encounter high pore pressure and so-called super reservoirs that further complicate borehole making.

In cases of initial drilling-in of high and low permeability Cenomanian reservoirs and sub-reservoirs with liner filter, the above problem may be solved by using lightened drilling muds with low fluid-loss rates for the construction of directional wells, vertical wells, and horizontal completions.
Field testing at the Urengoy and Yamburg fields demonstrated a reduced completion period and stable flow rates.

The compositions and properties of lightened, clayless, fresh and saline drilling muds based on sodium formate are provided below.
The first composition is a fresh, lightened, clayless drilling mud designed to drill in reservoirs with abnormal low pressure, high permeability and low clay content. Its components include: CMC-700 up to 1 percent, saponated tall pitch 0.3-0.6 percent, marble powder (fine) 2-3 percent, sodium tripolyphosphate (STPP) 0.01-0.015 percent, an antifoam agent 0.2 percent. Of those components, CMC-700 is used as a rheological and fluid loss properties controller, with marble powder employed as a filling agent to establish thin cake; there is also an antifoam agent, the saponated tall pitch (used as a surface tension controller to emulsify the drilling mud), a lubricant additive, and sodium tripolyphosphate to reduce drilling mud viscosity for the purposes of calcium ion neutralization.

The second composition is a lightened, saline drilling mud designed to drill in medium and low permeability and low clay content reservoirs with abnormal low pressure. Its components include: sodium formate 1-5 percent, carboxymethyl starch up to 3 percent, saponated tall pitch 0.3-0.6 percent, marble powder up to 3 percent, an antifoam agent 0.2 percent, lubrication additives up to 0.3 percent. The lightened, saline drilling mud contains sodium formate as a clay shale hydration protective agent, carboxymethyl starch as a rheological and fluid-loss properties controller, saponated tall pitch, a lubricant additive and marble powder. The composition also includes an antifoam agent.
The offered drilling muds ensure the maximum possible retention of the initial reservoir properties and prevent rock softening, both for sandstone and clay shales.

Technically, a result is deemed achieved if the following factors are ensured: high flocculating properties of a drilling mud used for disposal of cuttings during cleanout, high drill rate, and high heat-resistance of the drilling mud combined with low chemical aggression towards the environment when  the productive strata are being drilled in.

Under both medium and low permeability and high reservoir clay content, the saline drilling mud is used to preserve the reservoir's properties and to prevent swelling of a binding agent.

To provide quality drilling-in of productive intervals, the drilling mud's technological parameters have to conform to the following values:

• density, kg/cu. m  700-1,000 [1]
• relative viscosity, s 30-45
• fluid loss rate, cm/30 min 2-3
• gel strength, dPa/dPa 1 min/10??in 0-5/10-20
• cake thickness, mm 0.5-1.0
• cake friction factor as per FSK-2
device readout 0.1-0.15
• plastic viscosity, mPa*s 12-15
• yield point, dPa 50-80
• hydrogen index 8-9

Prior to logging, an antifoam agent is used to ensure the drilling mud's quality. In cases of sub-horizontal and horizontal completions, lubricant agents are added to reduce drillstring friction on the well wall.

[1] The drilling mud's density is modified based on actual reservoir pressure as per the Oil and Gas Industry Safety Rules and Regulations (Article 2.7.3.5).

want to know more?

Back