What is DRIMP?

Drilling Impedance (DRIMP) is a simple yet effective way of measuring the drilling efficiency performed by the drilling assembly. DRIMP can be calculated using downloaded drilling data, or may be calculated real time on the rig to provide an indicator of drilling efficiency. This methodology starts by employing the factual known outputs of the drilling process: rate of penetration (ROP), revolutions per minute (RPM) and weight on bit (WOB). These accessible figures are combined and calculated with normalization calculations of depth of cut (DOC), cohesion, internal friction angle and confining pressure to determine:

• Rock properties and porosity,
• Abnormalities while drilling, and
• Possible gas-bearing depleted zones.

This is done at real-time or near real-time in order for modifications to be made while drilling, virtually allowing the drill bit to become a tuned component of a logging-while-drilling system.

How does DRIMP differ from the Mechanical Specific Energy (MSE) method?

The development and application of MSE delivered a step change in performance analysis to the industry and helped the understanding of drilling issues that had not been fully understood previously. However, MSE requires at-bit torque information which is seldom readily available for analysis. Using surface torque provides an inaccurate account of downhole drilling conditions. Additionally, the MSE method uses calculations based on broad generalizations, not accounting for various cutting structure designs and differing IADC codes leading to inaccuracies. The introduction of DRIMP has eliminated these potential inconsistencies by providing a measurement that is based on the most basic real drilling data and is free of potentially inaccurate and generalized equation constants to provide an effective measurement of drilling efficiency. DRIMP delivers an elegant and simple solution to drilling optimization and has brought real benefits to operators around the world since its inception.

How does DRIMP fit into the Varel technology suite?

The patent pending DRIMP technology takes Varel’s GeoScience analysis to another level by providing the driller with a virtual roadmap of the wellbore. DRIMP performance may be correlated to the wellbore lithology and mechanical strength on an intervalby- interval basis to detect problem zones. This allows the driller to deduce whether or not drilling efficiency is optimal or if there are downhole issues impeding performance. For example, where a high DRIMP signature correlates with a hole section that exhibits a lower rock strength, this may indicate issues associated with below optimum hydraulics or interaction between the drill string and the sides of the wellbore further up the hole (figure 1).

Case Study 1: Identification of Hole Problems in Far East Wells

An operator in the Far East contacted the Varel team to investigate the cause of lower than anticipated drilling performance. A detailed investigation compared the GeoScience rock analysis to the DRIMP and found that intervals of high DRIMP did not correlate with where the bit encountered formation types with a higher mechanical rock strength signature (Figure 1). Further work carried out identified that the reduced drilling performance was due to the BHA engaging with the sides of the wellbore and hanging up in the caving zones further up the hole. Analysis with DRIMP and GeoScience enabled the operator to identify and understand the issue, as well as gave the driller the proficiency to recognize instances in which drilling performance is not optimized on future wells and develop a resolution.

Case Study 2: Detection of Depleted Reservoir Units

DRIMP investigation has been able to assist in the positive identification of depleted hydrocarbon reservoirs, where instantaneous drilling performance changes translated into DRIMP signatures to provide an immediate indicator of their presence. This can be confirmed after examination of drill cuttings at surface by the mud logging team and geologist, but DRIMP is not affected by the lag time for cuttings to be transported the often considerable distance from the bottomhole of the hole section to the solids control equipment at surface. In another Far East application the Varel team was approached to assess the technical feasibility of DRIMP to be used as an additional tool for the detection of depleted reservoir units. A key characteristic of depleted reservoir units is that the pore pressure of the units is typically less than that of the overlying caprock, which although impermeable, will typically have a significantly higher pore pressure due to the processes of basin subsidence and continued compaction from the overlying rock column over time. The change in the pore pressure gradient whilst circulating a mud system with a constant density when drilling into a depleted reservoir has a significant effect. The overbalance between the hydrostatic head exerted by the mud column against the pore fluid is significantly greater than the effective overbalance between the hydrostatic head of the drilling fluid and the non-depleted unit.

The net increase in the local mud overbalance between the drilling fluid and the depleted reservoir unit also increases chip hold-down whilst drilling. The phenomenon occurs when the removal of drill cuttings generated by the bit are subjected to a suction force as the differential pressure due to increased overbalance, thereby significantly reducing drilling efficiency in terms of ROP performance. However, analyzing ROP without normalizing for WOB and RPM may not provide a full understanding of the drilling dynamics. The introduction of DRIMP analysis (Figure 2) as an additional tool allows the drilling performance of the entire assembly to analyzed. Where ROP performance of the drill string changes, the DRIMP is able to factor in the effects of both WOB and RPM to normalize drilling performance. Without analysis of DRIMP it would be more challenging to determine whether ROP performance was a factor of modified drilling parameters or formation / pore pressure regime change.

Leading the Way in Drilling Optimisation

Varel International, Inc. is a leader in the field of advanced drilling solutions to meet the needs of your application challenges. Contact your local representative to see how our dedicated team of experts can help you optimize your drilling program.

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