Applying Depth Correction to Improve Deepwater Reservoir Development
Authors: Poedjono Benny , Nwosu Nnadozie , Martin Albert B 
Published: April 2019

The industry is making discoveries and drilling in areas and formations where along hole depth is of increasing importance. It is essential that the LWD depth meets positional objectives. The driller’s depth, which is the sum of the pipe strap measured while the pipes are on the surface, is used to calculate the logging-while-drilling (LWD) depth. However, environmental corrections must be applied to the driller’s depth resulting from the dynamic mechanical changes pipes undergo while in the borehole, with these corrections applied at the surface. These dynamic changes are due to drilling activities, temperature, and changes in the wellbore profiles and often result in LWD depth being shallower than the actual depth.

While drilling, dynamic and borehole conditions are known to significantly impact drilling operations and at best a block shift correction or depth matching to wireline depth is applied to the driller’s depth. However, an accurate drill pipe depth determination must include environmental corrections for the dynamic changes in the pipe stretch and compression, which vary with weight on bit, in addition to wellbore profile, torque, drag, friction factor, and borehole temperature.

In recent years, depth correction has been incorporated successfully in deepwater wells to environmentally correct for these errors and to improve the accuracy of the depth measurement. The challenge for drillers is overcoming the variations in depth measurements, to ensure wellbores are accurately and safely placed in the reservoir. Multiple techniques can be implemented to ensure this occurs, from drill pipe stretch modelling to depth measurements systems. The effect of depth correction has been observed in the multi-well pressure analysis for reservoir compartmentalization studies and fluid contacts.

Case studies are presented from wells drilled in deepwater where correction was applied to demonstrate its importance in reservoir development. In one case, the placement of pressure and sample points on the most accurate true vertical depth was achieved. By placing the pressure and sample points on the actual depth, a more precise assessment of sand continuity and oil/water contacts was obtained across the field. In another case, determination of the casing landing depth was obtained. In this case, correction was run in near real time to calculate casing stretch, helping to set the casing depth within the expected rathole while running in the hole. The effect of heave on depth on floating rigs often complicates image interpretation. One case study is presented to demonstrate improvement on the image after applying correction.

Modeling of depth uncertainties prior to drilling to understand the nature and magnitude of the correction is a novel approach and should be utilized in determining in the driller’s depth. The ability to maximize production and optimize drilling time requires LWD/MWD to play a significant role to get it right the first time. Accurate wellbore positioning allows for better reservoir exploitation, landing and setting casing depth, and understanding of the reservoir compartmentalization development risk.