Phosphonic Acid Complex for Stimulating HF-Sensitive Reservoirs – A Revolutionary Response

Abstract

HF-sensitive reservoirs that could not be hydraulically fractured nor effectively stimulated have been a challenge to the Petroleum Industry, hence locking the potential reserves that could otherwise be producible. These formations are typically those with high swelling clays, chlorite clays, feldspars and certain zeolites. Experience shows that even when such formations are stimulated with conventional HF- or HCl-based acid systems, the production performance declines shortly after the treatments, causing the reservoirs to produce at less-than-optimum rates. The skin damage associated with such reservoirs and stimulation by-products act as down-hole chokes, strangling production and increase drawdown. Successful stimulation of these kinds of HF-sensitive reservoirs had been carried out with a Organo-Phosphonic Acid Complex. Treatments have been carried out in seven Oil Producers, Water Injectors and Gas Injection wells in the Niger-Delta Nigeria and an oil producer in South East Asia. Post-treatment rates were above 800% of the pre-treatment values and more than 100% the maximum potentials ever recorded by most of these wells since they were commissioned. The new acid system offered true stimulation of carbonate and sandstone reservoirs as well as restoration to natural permeabilities of these intervals by the process of sequestration and complexation. An overall dramatic response of more than 200% over the wells treated with other formulations in the same basin was achieved. This paper discusses the non-HF based Organo-phosphonic acid formulations for Oil Producers, Gas Producers, Water Injectors and Gas Injection wells in both carbonate and sandstone reservoirs. The process of dissolution of the acid soluble and insoluble minerals that plug the near-wellbore region; candidate selection criteria; design; process of scale inhibition; field applications; results; treatment effectiveness and evaluations. The performance over other formulations is compared with respect to treatment costs, accelerated production, payback time, sustained production or Injectivity after treatment and percentage of original permeabilities being restored. Introduction The world's crude oil and gas come from limestone (CaCO3) / dolomite (CaMg[CO3]2) - Carbonate formations and quartz particles / Silicon dioxide (SiO2) - Sandstone formations. The carbonate formations could be found in their pure forms or in the form of carbonate or siliceous sands cemented together with calcareous materials whereas most sandstone formations are found in the form of quartz particles / silicon dioxide bonded together by various kinds of cementing materials, chiefly carbonates, silica and clays. Primarily, limestone and dolomite react at high rates with hydrochloric acid (HCl) and moderately with formic and acetic acids but sandstone reacts very little. The amount of reaction of these acids in sandstone formations depends on the amount of calcareous materials present. However, the silicon dioxide, clay and silt react with hydroflouric acid (HF). Since HF reacts with silt, clay, sandstone and most drilling fluids, it has been found effective in removing impairments and stimulation of sandstone reservoirs. The major defects of HF is the formation of by-products of calcium fluoride (CaF2) with calcareous material and sodium hexafluorosilicate (Na2SiF6), hydrated silica (SiO2·2H2O) and potassium hexafluorosilicate (K2SiF6) which are both insoluble and damaging precipitates. Carbonates, clays and iron compounds can ruin a well executed treatment. The chemistry of the reactions leading to the above insoluble and damaging precipitates are shown in appendix A. Fluorine is a very reactive element and since the composition of sandstone is varied, many reaction products can form when sandstone formations are stimulated with HF acid, hence HF-sensitive reservoirs that could not be hydraulically fractured nor effectively stimulated had been left unattended, thereby leaving the wells to produce at less-than-optimum potentials. Such reservoirs are predominantly made up of high swelling clays, feldspars, chlorite clays and certain zeolites.

Automating sandstone acidizing using a rule-based system

“…In one of the improved chemistry systems, HCl is replaced with a phosphonic acid complex which has five available hydrogen ions that dissociate at different stoichiometric conditions. For this reason, the phosphonic acid complex is referred to as a five-hydrogen (HV) complex (Nwoke et al 2004;Uchendu and Nwoke 2004;Rae and Di Lullo 2007). The HV acid reacts with ammonium bifluoride (NH 4 HF 2 ) or with ammonium fluoride to produce HF acid.…”

“…Water-wet condition improves the acid contact with the targeted alumino-silicate material. Precipitation of fluorosilicates, hexafluorosilicates, alumino-fluorosilicates, iron compounds and calcium fluoride, commonly generated during acidizing with conventional HF are prevented as a consequence of the strong chelating property of the HV:HF acid system (Nwoke et al, 2004). These numerous properties of the HV:HF acid system are, indeed, the reason for the improved success rate of acid jobs in several case histories.…”

“…Acid formulation requires careful mineralogical analysis of core samples. For instance, a sandstone formation containing authigenic iron chlorite clay within its pore spaces, even with low volume fraction, may not respond to treatment with HF in any concentration, and can be detrimental in response to HCl treatment as well (Nwoke et al 2004). On the other hand, traditional guidelines, based on bulk mineralogy, might suggest treatment with mild or moderate strength HF for low total chlorite content regardless of the clay distribution in the pore space (Coulter and Jennings 1997).…”

CT-Deployed Multipurpose Nozzle for Effective Sand-Cleanout and Acidizing Operations in a Single Trip: Injector Wells Case Histories

“…This formulation combines matrix acidising and scale inhibition technology; removal of acid soluble scales and prevention of re-precipitation. The lack of HF reduces the danger of damaging precipitates, while the simplicity of the system makes it ideal for stimulating long intervals or multiple zones in a single operation 7,4 .…”