Measuring and Using Shale Density to Aid in Drilling Wells in High-Pressure Areas
Authors: Boatman W.A. 
Published: December 2007

Continuous bulk density tests on shale cuttings while drilling have shown reverse anomalies in normal increase in shale compaction. These reversals from normal density of shale indicate the entrance into abnormally high pore pressure shale. Deviation numbers from the normal density can be used to calculate both accurate mud weights as drilling proceeds and a more advantageous casing point in deeper drilling.

Introduction

Myers and van Siclen were the first to publish work to demonstrate from actual shale density tests on drilling cuttings the anomalies in shale compaction with depth in the Trull field, Matagorda County, Tex. They said that shale compaction does not increase regularly with depth as previously thought by Athy and Hedberg. Also, sands associated with reversals in density possessed pressures considerably higher than normal hydrostatic pressure. Dickinson recognized this and also stated that elevated interstitial fluid pressures were principally caused by compaction where low shale permeability prevented water from completely escaping.

Several papers have recently been written on identifying high-pressure shale zones from electrical surveys by using resistivity, conductivity, sonic and density log data. Points chosen from these logs for calculations must be from clean shales without the masking influence of a calcareous, silty or sandy shale. Close proximity to sand bodies from which these values are taken also affects the calculated points. These lithological characteristics also affect the general trend line of physical bulk-density analyses, although the fluctuations are offset by the larger number of analyses consistently taken as drilling progresses.

According to Rogers, Shell Oil Co. uses a drilling rate system of anticipating high-pressure shale zones by noting trends toward increased penetration rates. This is confirmed by resistivity and sonic logging.

This article not only confirms Myers and van Siclen's anomalies in shale compaction by bulk density measurements, but also presents a field application for indicating the presence of low-density, over-pressured shales while drilling and predicting the approach to these dangerous zones. By reducing the analysis time from approximately 15 hours to less than 10 minutes, a continuous plot of density trends may be kept as close to the bit as the lag time of the mud column.

The main objective of this type of logging is safety, but three functions emerge as being particularly important to a cost-conscious industry.By setting casing into the over-pressured shale so that, as mud weight requirements increase with depth, no open hole having normal pressure will be exposed to the possibility of breaking down the formation. Proper use of shale-density plots with other tools may often save at least one string of pipe.As long as a normal trend of compaction is being plotted, drilling should proceed with as light a mud as other drilling indications deem prudent the lighter the mud, the faster the hole can be made.When a low-density shale is penetrated, mud weight requirements can be calculated from density plots. These mud weights are based on interstitial fluid pressures and must be increased according to hole size and swabbing effect of pipe trips.

Although the presence of shale with abnormally low densities has been noted throughout the Texas-Louisiana Gulf Coast, wells selected for study were logged from Cameron to LaFourche Parishes, La., and particularly offshore.

Shale Density Analysis Procedure

Several approaches of analysis and instrumentation used at the start of this work were subsequently discarded due to weaknesses in methods or in poor field operations. Such systems as liquid displacements, liquids graduated in specific gravity and even a scale based on Archimedes principle gave varying data depending on interfacial tension, gas trapped in shale particles, temperature and irregular sample surfaces. Also, the field use of many of these techniques was undependable. The bulk volume measurement was finally accepted and proved reproducible to within less than 1 percent accuracy.

JPT

P. 1423ˆ