ECD Management Tool Offers Potential for Reducing Drilling Problems and Costs
Authors: Hariharan P.R. , Judge Bob 
Published: October 2008

As offshore drilling operations advance to deeper water and well depths, one of the primary problems faced during circulation is excessive equivalent circulating densities (ECD). This problem becomes acute in the case of extended reach wells, especially where the margins between pore and fracture pressures are narrow. Excessive annular frictional pressures cause a variety of problems ranging from wellbore instability, lost circulation and stuck pipe, apart from limiting the depths to which drilling can be carried out without approaching the formation fracture strengths. Several initiatives have been undertaken or are presently underway to manage this problem.

This paper presents a means to manage the ECD using a subsea pressure monitoring and control device that is located at the mudline. The return mud is directed through a subsea control device capable of applying back pressure to the well. The bottom hole pressure in the well is automatically managed and controlled during drilling and circulation utilizing fieldproven devices. Maintaining bottom-hole pressure in this manner creates options to utilize lighter weight drilling fluids while still preserving desired safety margins.

Analysis of pore and fracture pressure data and casing configurations for several wells illustrates that it would be possible to drill with lighter drilling fluids along with an applied back pressure. The ability to apply back pressure to a well, creating a "virtual"mud weight, allows faster influx detection and initiation of control procedures compared to the conventional method of weighting up drilling fluids. Other potential advantages include increased penetration rates, decreased possibilities of differential sticking, reduced incidents of wellbore 'ballooning' during connections as well as the ability to differentiate between formation influxes, and decreased formation damage, all of which enable wells to reach TD at lower costs.

The paper discusses the results of well data analysis that indicate managing ECD in this manner would widen the drilling operation window. Also discussed are the equipment and operating details of the proposed system, as well as various methods historically employed to reduce ECD. Finally, a discussion of the pros and cons of deploying a system at the seabed compared to one at the surface is presented.

Introduction

It is well known that in the case of offshore wells where the pore-frac windows are narrow, managing the equivalent circulating density (ECD) becomes extremely important as well as challenging. In the presence of these narrow windows, precise management of the ECD is the difference between not fracturing the formation and not allowing influx from the well. The ECD is a function of a combination of the mud weight (MW), the rheological properties (especially the low and high shear-rate viscosities), the frictional pressure drop in the annulus and solids loading. There are abundant references in the literature which cite that the management of circulating pressures ECD related problems becomes more acute in the case of extended reach wells (ERD). In high-temperature and high pressure wells (HTHP) it often becomes very difficult to redict the ECD based on some assumed mud properties which end up either over- or under-predicting the actual values. Unfortunately, in wells with narrow margins both situations end up being potentially disastrous; causing either a lost circulation scenario or a well-control situation.