Comparative elemental, mineral and microscopic investigation of sandstone matrix acidizing at HPHT conditions

Abstract

Matrix acidizing is usually conducted on sandstone reservoirs to increase hydrocarbon production. Mineralogy plays an important role in designing acidizing treatments to enhance the production of oil and gas from sandstone reservoirs. The most common acid in practice to acidize sandstone formation is mud acid after the pre-flush acid stage. The main aim of the pre-flush acid stage is to dissolve positive ions like calcium, potassium, sodium, etc, which can cause precipitation reactions during the mud acid stage. This research is aimed to investigate the reaction mechanism of a new pre-flush acid combination (hydrochloric acid and acetic acid) with sandstone formation. Core flooding experiments were performed to react acid with the core samples (6 in x 1.5 in) under high pressure and temperature (HPHT) conditions (80 °C and 1000 psia). Analytical techniques such as Tescan Integrated Mineral Analysis (TIMA) have been used to illustrate the effect of these acids on sandstone formation. TIMA analysis showed that the new acid combination was effective in dissolving various minerals (ankerite, magnetite) and cations (calcium, magnesium, sodium, and iron). 15% HCl proved to be more effective in the dissolution of different particles from sandstone core samples. It dissolved 32.8% of the initial number of particles present inside the sample while the dissolving power of 5% CH3COOH:10% HCl is 26.7%. Subsequently, the number of pores increased by the application of 15% HCl is 22% while that by 5%CH3COOH:10% HCl is 21.8%. Density analysis showed that both acid combinations removed heavy particles effectively, whereas 5% CH3COOH:10% HCl proved more efficient in dissolving calcium ions which is very important for pre-flush acidizing.

Keywords

• Mineralogy
• Sandstone
• HPHT
• Core flooding
• TIMA
• Hydrochloric acid