An Innovative Processing Method for High Resolution LWD Density Images in Water and Oil Based Mud

摘要

With the rapid growth of horizontal drilling, azimuthal LWD bulk-density images (and also photo-electric effect images) have proven to be indispensable tools for the identification of bed boundaries, estimating bed dips, and determining the steering direction of the drilling assembly. The inherent low resolution of the density measurement has, however, typically limited the interpretation of these images to the analysis of structural scale features. The abilityto image finer scale detail is governed to a large extent bysampling density. Sampling density depends upon the sampling frequency of the instrument, the rate of penetration of the drilling assembly and the rotary speed of the drillstring. Conventional downhole image processing schemes average raw measurements either in time or depth before sectoring them into relatively large circumferential sectors. This averaging process conserves tool memory and smooths noisy data but inevitably results in the loss of high spatial frequency instrument responses associated with fine scale geological features. Instead of compressing data, we present a method that preserves all the raw measurements stored in the tool memoryand maps them to a grid around and along the borehole. Statistical noise is mitigated and a specially designed interpolation scheme is used to fill empty grid locations before analyzing the data and smoothing the results. This methodology allows the creation of high resolution images with up to 256 circumferential sectors and a depth increment as small as 0.6 inches. The technique is equally applicable to borehole image data acquired by any type of logging tool providing that the raw measurements are frequently sampled and stored in the tool memory. For example, high resolution borehole caliper images have also been created from ultrasonic transducer measurements of the same tool used to acquire the data presented in this paper. Azimuthal bulk-density data acquired in this manner allows for the opportunity to produce optimized images in oil-based mud and overcomes the limitations of LWD micro-resistivity imaging tools in non-conductive borehole fluids. This technique has been successfully applied to wells drilled in both the UK North Sea and US land. Field examples presented in this paper illustrate the benefits of the application in imaging thin beds, sedimentarybedding structures, coal seam bedding internal structure, faults and fractures.