Rig-site monitoring of the solids content of drilling fluid and discharges from solids control equipment
Authors: Davison J.M. , Daccord G. , Prouvost L. , Gilmour A. 
Published: December 2007

The nature and content of suspended solids in drilling fluids directly affects its key properties, adversely impacting the drilling process, which in turn lowers overall well quality, augments costs and increases the environmental impact of drilling. To control the impact of the drilled solids on drilling fluid properties, the engineer at rig-site firstly requires an effective monitoring tool, allowing frequent and accurate determination of the solids content at various points in the circulating system.

A technique based on X-ray fluorescence (XRF) has been developed which is being utilised at the rig-site to overcome the limitations of the traditionally employed methods. It is the aim of this paper to document the development of the XRF technique and its implementation at the rig-site. XRF spectra from a sample are input into a multivariate calibration model to predict the concentrations of solid phases (barite, low-gravity solids), liquid phase (water or oil and brine), as well as some ion concentrations (potassium and chloride in water based mud, chloride in oil based mud). The accuracies of the predictions for the solid phases are significantly better than the traditional methods.

The ease and speed of use of the XRF technique facilitate multiple sampling from various points in the circulating system and effluent discharges. Two case histories are cited which demonstrate the use of the XRF technique for monitoring the deployment of solids control equipment at the rig-site and optimising its configuration.

Introduction

For any hole section to be drilled, the chosen drilling fluid has specific properties to be maintained for efficient, cost-effective, drilling. The fluid density, viscosity, gel strengths, filter cake properties, inhibition levels and lubricity are all adversely affected by the drilled solids entrained during the drilling process. As a consequence of the increased solids loading in the fluid, the performance of the fluid deteriorates resulting in decreased rates of penetration (ROP), decreased hole-cleaning efficiencies, increased chance of differential sticking and increasing the chance of solids plugging producing formations, therefore impairing future well productivity. In managing and controlling the solids content of the drilling fluid, the engineer at rig-site requires an effective monitoring tool, allowing frequent and accurate determination of the solids content in the circulating fluid and also effluent discharges from solids control equipment (SCE).

A technique has been developed which is being utilised at the rig-site to overcome the limitations of the currently employed methods. The concept of measuring the solids in drilling fluids by X-ray fluorescence (XRF) was presented in 1993 (ref. 1). This paper described the evaluation of this technique using a model calibrated with water based muds (WBM) comprised of barite, calcium carbonate to simulate drilled solids and brines of various salts. The model used XRF spectra coupled with certain sample specific inputs to predict the concentrations of high gravity solids (HGS), i.e. barite, low gravity solids (LGS), water and salt. Contamination experiments verified that substituting LGS phases of different chemical composition did not affect the predictive performance of the model. The quoted accuracy of the model was 0.32 volume percent (v/v%) for HGS and 0.8 v/v% for LGS. The resolution by which variations in LGS content could be detected was demonstrated to be approximately 0.3 v/v% absolute. It was estimated that the XRF technique was some ten times more precise than the traditional API retort technique for calculating solid volumes in drilling fluids.

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