Testing Protocol And Breakeven Cost Analysis of Mud Dewatering For Closed-loop Drilling System
Authors: Wojtanowicz A.K. 
Published: March 2010

Closed-loop drilling operations in environmentally sensitive areas require integration of conventional solids control system with chemically: enhanced centrifugation, a continuous dewatering process. The paper presents a design method for continuous dewatering. The method-is based on rigsite mud dewaterability testing and breakeven cost economics.

The protocol for dewaterability test describes instrumentation and stepwise procedure to measure, dewatering cake's moisture; content, and evaluate efficiency of volume reduction Emphasized is preparation of weighted mud samples through removal of weighting material and correction for residual barite. The presented testing protocol eliminates volumetric measurementsand replaces them with weight measurements, which improve accuracy by an order of magnitude.

The paper explains how to use results from dewaterability tests to analyse cost efficiency of the dewatering component inclosed-loop drilling operations. The analysis is based upon an analytical model of breakeven cost The model combines dewaterability with depth related quantities representing drilling progress and rates of waste generation; recycling and disposal The paper shows:How to evaluate dewatering -performance; while drilling;How to set up technical requirements soliciting bids for dewatering services; and,How to predict drilling depth below which the process is not economical.

Introduction

Continuous dewatering of drilling mud is a well site process for separating and recycling the mud liquid phase and minimizing the volume of drilling waste. The process has been already patented(1) and used in field operations(2, 3, 4). The analysis of published reports from these applications shows that economic performance of the dewatering process depends primarily upon three factors:high cost of off-sit~ disposal;restricted disposal on-site (a ban on the annular injection, for example); and,the degree of volume reduction attained thorough dewatering(5).

Therefore, any successful implementation of the dewatering process requires testing the mud dewaterability and combining the test results with cost factors to analyse process economics.

Dewaterability is the ability of a drilling fluid to release its water phase if subjected to mechanical expression. Before the expression, the drilling fluid has to be chemically conditioned in order to destabilize the suspension and free the water phase. (The design of chemical conditioning and its optimization has been described elsewhere(6).) Like other characteristics of drilling muds-the API filtration, for instance-dewaterability results from complex physical mechanisms but can be determined simply by measuring relative volume of released water and moisture content of the dewatering drilling fluid to calculate volume reduction efficiency EVR. The objective of this paper is to present the dewaterability testing procedure.

Currently, the only method for evaluation of dewatering performance is based on post-well or monthly recaps. In this method, cumulative volume of waste drilling mud is used to average cumulative expenditures and calculate the cost of dewatering per barrel of waste mud(7). Then, this cost is compared with the hypothetical disposal cost of the same barrel without using dewatering process. Finally, a conclusion is made whether the use of dewatering would result in savings or losses. Although correct in principle, the method is deficient because it does not help to design the process or evaluate its daily performance.