Precipitation During the Acidizing of a HT/HP Illitic Sandstone Reservoir in Eastern Saudi Arabia: A Laboratory Study

Abstract

Field results indicate that the Jauf reservoir, a deep, high temperature illitic sandstone, is sensitive to conventional mud acid treatments. As a result, an extensive study was performed to better understand the problem. The focus of this paper is on core flow tests using various inorganic and organic preflushes and mud acid (HCl - HF) systems. Interpretations of the core tests results are supported by chemical, XRD, SEM, and Thin Section analyses. A detailed description of the mineralogy of cores prior to and following an acid treatment is presented with emphasis on precipitation of reaction products, fines migration and other potential damage mechanisms. Chemical analyses indicated that the HCl preflush extracted the majority of the aluminum during a conventional mud acid treatment. As a result, the Si/Al ratio during the subsequent mud acid stage is a false indicator of hydrated silica precipitation. Additionally, precipitation can occur in acid effluent collected during a core test once the fluid cools to ambient temperature. This results in an erroneous Si concentration and impacts numerous calculations and "conclusions" i.e., reaction kinetics, amount of silica precipitation, etc. The same analogy applies to flowback samples that are used to postulate various downhole phenomena. Although potassium hexafluosilicate was present in core effluents, it was not present in the cores based upon XRD, SEM, and Thin Section examination. Potassium hexafluosilicate was identified as two distinct crystal structures i.e., cubic and pyramidal. SEM examination showed that most cores exhibited severe alteration of illite and chlorite to yield a significant quantify of clay residue including fines. The residue, severely degraded potassium feldspar (microcline) and quartz fines created formation damage subsequent to treatments with 12 wt% HCl – 3 wt% HF mud acid. This paper supports the literature in some cases yet spotlights some possible flaws in our understanding of sandstone acidizing. Introduction Wells completed in the pre-Khuff (Jauf) reservoir have the potential to deliver a significant quantity of sweet gas. The Jauf retrograde gas reservoir, located in eastern Saudi Arabia, exhibits a bottomhole temperature from 280 to 300°F and is normally greater than 14,000 feet deep with 8600 to 8800 psi reservoir pressure.1 Table 1 indicates the Jauf reservoir in Well "A" is primarily composed of quartz, but contains significant amounts of potassium feldspar (microcline) and clay minerals including illite, chlorite, and illite/montmorillonite mixed-layer clay. Note greater than 95 wt% of the mixed-layer illite/montmorillonite clay is illite. This complex mineralogy suggests that the formation may be "sensitive" to aqueous fluids i.e., damaged during drilling, completion and workover operations. This is supported by field and laboratory experience. Previously, it was reported that the Jauf reservoir was "water-sensitive" and easily damaged by fresh water.2 Lynn concluded that a minimum concentration of 5 wt% KCl was required during drilling to minimize formation damage. Field experience complements Lynn's work i.e., a well completed in the Jauf reservoir was drilled with a low chloride content mud and perforated in a fresh water mud to yield a skin damage of greater than thirty. An attempt to remove the formation damage incorporating a 15 wt% HCl preflush to 12 wt% HCl – 3 wt% HF (mud acid) was not successful and production decreased by 40%. This study was initiated to address damage removal in the Jauf formation.

Acidizing Sandstone Formations Using a Sandstone Acid System For High Temperatures

“…Among these problems are: decomposition of clays in HCl acids, the presence of carbonate can cause the precipitation of calcium fluorides (CaF 2 ), HCl-sensitive clays (eg., illite) can cause formation damage, the high reaction rate and corrosion rate at high bottomhole temperatures (Shuchart and Gdanski 1996;Thomas et al 2001), and mixing between various stages of the treatment may occur during the treatment by mud acid (Gdanski and Shuchart 1998).…”

Optimization of Production Gas Wells by Acidizing Using PLT Interpretation in Hassi R'Mel Field, Algeria

“…Indeed, in contact with chlorite, there is the formation of fluosilicates aluminium. The presence of anhydrite, after its hydration with acid, leads to the formation of gypsum, which also reduces the porosity (8,9) .…”

Core‐Scale Experimental and Numerical Investigation on Fluoroboric Acidizing of a Sandstone Reservoir

“…[3,4] Field application showed that most well productioni ncreased significantlyi nitially after the mud acid treatment but then declined rapidly, and some wells had no response or even productionl oss.L aboratory and theoretical studies indicated that the unsatisfactory results can be because of the rapid reaction rate of the acid with clays and silicates,shallowpenetration and strong corrosion of the sandi nt he near wellbore area, and precipitation of reaction by-products and migrating fines that plug the pore space. [5][6][7] To overcome the drawbacks of conventional acids,r esearchers have dedicated great effortst of ind other acid formulations,a nd the fluoroboric acid system was studied intensely among severalc andidate acids.C ompared to mud acid, the greatest advantage of the HBF 4 formulationi s the greater acid penetrationd istance. In ac ertain temperature range,t he process that fluoroboric acid hydrolyzes to generateh ydrofluoric acid is relatively slow,w hich leads to as ignificant increase of the live acid penetrationd istance.…”