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Acidizing a Sandstone Formation Successfully in the Gulf of Cambay

Fig. 1—Gulf of Cambay.

This paper describes a matrix-acidizing campaign executed successfully in the Gulf of Cambay on the west coast of India. In initial laboratory tests and during simulation runs, the goal was to design a preflush/acid/post-flush system to best suit challenging reservoir conditions while also considering offshore logistics. After pumping the system in one well, the system design, pumping procedures, and volumes were modified to improve results further in the next well.

Introduction

The offshore field, located in the Arabian Sea off the western coast of India, has been on production since 1997 (Fig. 1 above). The field covers an area of 1471 km2 and lies 160 km north/northwest of Mumbai. The reservoir consists of a stacked series of sands deposited in the Lower Miocene and Oligocene. The field has up to 13 different Olio-Miocene gas-bearing sands separated by shales. Reservoir-sand permeability ranges from 100 md (in shaly beds) up to 5,000 md in clean sand beds. Because of the unconsolidated nature of the reservoir sands, gravel-pack screens are a typical well completion in the field.

Mineralogy

The formation can be described as iron-rich sandstone, with average iron-bearing chlorite content across 20 samples of 18%. Chlorite is a ferrous--dominant, minor ferric mineral and is partially soluble in hydrochloric acid (HCl). Acid dissolution of chlorite, an iron-bearing mineral, could create pore-plugging iron hydroxide precipitates. In the presence of chlorite clays, an increase in the iron-control-additive concentrations in the HCl preflush is recommended. Previous guidelines suggest using 5% HCl or 5% acetic acid as a preflush for sandstone matrix acidizing in chlorite-rich formations. On the other hand, this rock type contains a significant amount of hematite that occurs as the oxidation product of iron-rich detrital grains. Acidizing the ferric iron-rich hematite with HCl-based blends should be expected to generate ferric hydroxide gels, which could plug reservoir pores and pore throats, thereby lowering productivity.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 164631, “Overcoming Challenges While Acidizing Sandstone Formation Successfully in the Gulf of Cambay, Offshore India,” by Sergey Stolyarov and Anwar Alam, Baker Hughes, prepared for the 2013 SPE North Africa Technical Conference and Exhibition, Cairo, 15–17 April. The paper has not been peer reviewed.
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Acidizing a Sandstone Formation Successfully in the Gulf of Cambay

01 June 2014

Volume: 66 | Issue: 6

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