Abstract

The mechanical friction generated during drilling operations can be problematic in long, narrow, deviated and highly inclined wells. Efforts to reduce the excessive friction include application of synthetic-based muds or oil lubricants.

However, these fluids are highly restricted because of environmental concerns. Even adding small amounts of oil-based lubricants to water-based fluid can raise environmental and formation damage concerns.

Shaker blinding issues were observed while drilling a heavy oil reservoir in the Peregrino field offshore Brazil. Based on field observations, the blinding was thought to be caused by a combination of the heavy crude oil, drill cuttings, and mud lubricant. An extensive laboratory study included simulation of screen blinding, a compatibility study between mud lubricants, heavy crude oil and drill cuttings, and, ultimately, a proposed solution based on a combination of mud additives to help to mitigate the problem.

During this study, several lubricants and combinations of additives were subjected to a sensitivity study based on technical and environmental requirements to select an optimized and customized solution. The study included friction coefficient determination to compare the performance of proposed combinations and actual drill-in fluid being used. For the screen blinding simulation, a mechanical shift sieving apparatus was used. An unconventional lab test was adapted from existing completion fluids literature for the compatibility study.

The study also included shaker screen visual and microscopy analysis, as well as analytical chemistry laboratory determination of the nature of screen blinding with elemental analyses, and Fourier transform infrared spectroscopy (FTIR). Grease, sieve, and emulsion potential tests were run using liquid additives from mud formulations described in previous lab testing for the screen blinding issue. For the emulsion potential tests, formulations were treated with 1% v/v crude oil and combinations of 1% v/v lubricants and solvents in an attempt to prevent/mitigate possible interactions of these components with the crude oil.

A lubricant cocktail provided the best performance in the lubricity test, improving the lubricity coefficient from 0.303 (blank) to 0.15, with a torque reduction of 55.8%. The solution contained no grease and was formulated using environmentally acceptable components with minimal formation damage risk.

Introduction

The Peregrino field is located in Block BM-C-7 of Brazil's southwestern Campos basin, about 85 km offshore Rio de Janeiro. Once fully developed, the field will consist of two drilling platforms, set in about 100 m (330 ft) of water and tied back to a centrally located floating production, storage, and offloading (FPSO) vessel. Oil is produced and transported from the platforms by means of subsea flowlines to the FPSO for processing and storage. The Peregrino reservoir is located about 2300 m below the seabed in relatively heterogeneous, high-permeability (6 to 15 darcies), unconsolidated sands that are interbedded with shales/mudstones. The Peregrino crude is heavy (No. 13. API-0.98 SG) and of high viscosity (164 cP at downhole temperatures). The temperature of the reservoir is 80°C. The field development plan consists of approximately 60 wells to be completed in three phases.

Background Analysis

Shaker screen blinding problems have been observed while drilling the reservoir interval in Peregrino field, especially in Drilling Platform A. The problem seems to be caused by accretion of drill solids and crude oil from the formation, probably triggered by the use of certain drilling fluid lubricants.

The approach for the study of the problem to find a potential solution included the following:

  1. Identify the physical-chemical nature/composition of the sludge through drilling fluid lab and chemical lab analysis.

  2. Lab simulation of the screen blinding process.

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