Development of New Kill Fluids with Minimum Sagging Problems for High Pressure Jilh Formation in Saudi Arabia
INTRODUCTION Barite sag is an unwanted variation in drilling fluid density after an extended period of static conditions. This problem is experienced frequently in Khuff and pre-Khuff gas wells in the Base Jilh Dolomite (BJD) formation. The Jilh formation is a dolomitic Middle Triassic tight zone that ranges from 8,000 ft to 10,000 ft true vertical depth (TVD) with abnormal saltwater pressure. Several factors add further complication to the problem of well control, such as uncertainty in predicting pore pressure, high temperature (300 °F), well deviation (leading to the Boycott settling phenomenon), drillpipe eccentricity and salt contamination of the drilling fluid. This high-pressure BJD formation requires a drilling fluid with a density up to 150 pcf. For high density fluids, achieving the right fluid formula with the right fluid rheology is difficult since excessive heat, dehydration and solids loading can deteriorate mud properties. The addition of weighting materials, such as barite (BaSO4) or hematite (Fe2O3), is needed to achieve the desired density to overbalance the well and avoid any possible flow from the formation; however, maintaining a high volume of solids content — of BaSO4 particles or any other material — in suspension is challenging, especially for extended periods of time. The separation of weighting material particles — usually BaSO4 — from the mud leads to undesirable density fluctuation as the drilling fluid is being circulated throughout the hole. This phenomenon, often referred as barite sag, may lead to serious well control incidents, stuck pipe, lost circulation and nonproductive time. Previous experience has shown that the barite sag phenomenon is attributed not only to mud-related problems but also to operation related practices1. Drillpipe eccentricity, stationary drillpipes, hole inclination and low annular velocities can dramatically increase barite sag2. Barite sag increases even faster as the hole angle increases to between 45° to 60°. This process was reported by Boycott in 1920, when he noticed that blood cells settled faster in inclined test tubes than in vertical tubes3.
Mitigation of barite sagging during the drilling of high-pressure high-temperature wells using an invert emulsion drilling fluid
Abstract Sagging of particles is one of the most critical issues when drilling deep oil and gas wells especially when the hole angle is greater than 30°. Accumulation of cuttings at the bottom of the hole around the drilling bit as a result of barite sagging cause severe problems such as the loss of well control. Temperature has a major effect on barite sagging so there is a need to develop a new drilling fluid formulation that can prevent barite sagging under different geometries of hole path and high temperatures (greater than 300 °F). The objective of this study is to evaluate the effect of using an organophilic phyllosilicate (OP) as an anti-sagging additive to prevent barite sagging in an invert emulsion drilling fluid (IEDF) at high temperature (350 °F). Static sag tests were conducted at vertical and inclined (45°) positions at 350 °F. Dynamic sag tests were performed at 120 °F. The second objective is to determine the optimum concentration of OP and evaluate the effect of the selected concentration on the rheological and filtration properties of the IEDF at 350 °F. The results show that the optimum amount of OP for preventing barite sagging at 350 °F in both vertical and inclined positions is 1.5 g. Adding 1.5 g of the OP prevents barite sagging under dynamic conditions and enhances the rheological properties by increasing the yield point plastic viscosity ratio. The OP enhances the suspension capability of the drilling fluid at low shear rate (3 rpm).
Role of Rheology in Barite Sag in SBM and OBM
Barite sag in SBM and OBM is affected by drilling fluid properties and drilling parameters such as flow rate, hole geometry, pipe trips, and others. Of these, fluid rheology has received the most attention and is perceived to be a dominant and complex factor affecting sag. Available guidelines for mitigating sag, derived mostly from measurements under laboratory conditions, relate barite sag to the low-shear-rate rheology of the fluid as measured by oilfield-type rotational viscometers. The guidelines have been used with varying degrees of success to reduce sag in invert emulsion fluids. Frequently, however, they are inadequate for eliminating or reducing sag to acceptable levels. Apart from other non-rheology related parameters, a main reason for this is the inadequacy of current rheology guidelines for application across a wide range of fluids. This may be due to the fact that the low-shear-rate characteristics of some fluids are poorly defined by conventional oilfield viscometers, or that the fluid structures responsible for solids suspension are better defined by linear viscoelastic properties. This paper discusses rheological properties that characterize the degree of structure in invert drilling fluids. It presents steady shear data at very low shear rates and linear viscoelastic properties for a range of fluids formulated with different viscosifiers. The data show that dynamic sag best correlates with certain viscoelastic properties. Such correlation is not evident from the steady shear data.