TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe use of unique or modified HF acid systems for matrix acidizing within the Niger Delta and some other parts of the world is still the preferred method for effective stimulation of sandstone reservoirs. The continued success of treatments done with modified HF systems in this area are due to its ability to mitigate the rapid spending of the active acid with clays and silicates; prevent matrix unconsolidation in the near wellbore region and the subsequent precipitation of acid reaction by-products within specific well applications. This success has brought about modifications to the acid system and new applications for various well scenarios. This paper discusses the successful application of a unique sandstone acidizing process that uses a single acid stage (no preflush or postflush is required) making multiple stage treatments much simpler while still achieving both economic and technical targets for the well. It covers the non-formation stimulation treatment of several wells using this single-step method and also describes a successful approach to improving the productivity of high water cut wells with severe finesdamaged gravel packs by this system. Unique experiences acquired during the field trials carried out within the Niger Delta using this system are evaluated. The results demonstrate a significant increase in incremental hydrocarbon production for normally high risk, low reward wells and remarkable stimulation cost reduction via reduced chemical volumes, less equipment requirements and shorter job times.Also included are a thorough analysis of actual candidate selection criteria, fluid chemistry, actual job design and operational issues during execution of treatments. Key technical and economical performance indicators including skin factors, production rates, specific productivity indices, treatment costs indicate that this system has certainly and successfully increased the application envelope for unique HF treatments within sandstone reservoirs.
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.
The formation of petroleum residue and deposits is a recurrent problem in low API gravity or heavy oil reservoirs and reservoirs with significant pressure and temperature fluctuations. It is also a challenge in fields where recent well interventions has pushed pipe dope into the perforations resulting in severe loss of native formation permeability and productivity. This paper discusses the application of a unique solvent acid dispersion that offers one step stimulation of reservoirs damaged with both plugging solids and organic deposits. Conventional acid systems including the numerous patented HF - HCl and organic acid blends dissolve the plugging solids and cementitious materials but cannot stimulate sandstone reservoirs damaged with organic deposits or a combination of plugging solids, fines migration and organic deposits. In fact, the formation of acid induced sludge is common. However, this acid - solvent blend dissolves asphaltene and paraffin deposits, prevents the formation of wax crystals and removes organic residue and inorganic materials simultaneously. A special surfactant train acts synergistically with the acid blend, prevents acid induced sludge and leaves the rock surfaces strongly water wet. Overall treatment efficiency was increased especially in gravel packed intervals by using a rotating jetting tool although success has also been recorded using the bullheading option. Recent experience has shown the acid blend is particularly suitable as a spearhead for injection wells. Optimum solvent loading for effective stimulation is between (10 – 20) percent depending on reservoir temperature, suspected damage mechanism and the severity of relative permeability effects. Unique experience detailing standard lab / field practice and exceptional results for several wells in the Niger Delta is the focus of this paper. Wells treated with this unique blend either as spearhead for other specialty acid systems or as a one step treatment show dramatic and sustained productivity increases of between (50 – 150) % relative to initial well data. The system has been proven to be successful in most types of completion architecture. Key technical and economic performance indicators including skin factors, production rates, specific productivity indices, payback time and treatment cost indicate this low viscosity acid blend is both technically proficient and cost effective. Introduction and Justification The in-situ remediation of damage caused by organic material deposited in the near wellbore region from well construction and intervention operations i.e. from POBM's, pipe dope, e.t.c. And from the producing system itself (waxes, resins, and asphaltenes) has historically been tackled using either one or a combination of the following measures: Hot oil - water treatments, Hot water - solvent combination, Solvent - surfactant - dispersant trains and crystal modifiers.1 The use of hot oil / water treatments is useful in the treatment of surface equipment clogged up with wax. Though these measures have attained a given degree of success even in reservoir stimulation applications, their continued usage has been proven to concentrate higher melting point carbon molecules in existing deposits, hereby making the resulting deposits more resistant to further heat treatments. This method has also been shown to cause significant formation damage by the way of negative relative permeability effects. Solvents and solvent / surfactant /dispersant trains are excellent means of organic damage removal in the reservoir. However, the extent of their organic carrying abilities is limited by temperatures below the cloud point of the organic material / solvent combination. Large quantities of solvents are typically required i.e., 100 lbs. of xylene will just dissolve 6 lbs. of C36 paraffin at 100°F.2 However, the use of the proper combination of chemicals (determined by proper lab tests) can reduce treatment volumes and costs considerably. The use of crystal modifiers or pour point depressants provide a more effective means of preventing deposits in the reservoir but are usually cost prohibitive.
Wells requiring combined sand clean out and acidizing operations have been routinely treated using BHA's containing different nozzles in multiple trip operations. This is especially true for sanded-up completions. The normal scenario would be to use a standard wash nozzle for both acidizing / sand cleanout operations or to use a switchable nozzle for the sand clean-out, POOHpooh and then run a rotating jetting nozzle (for acidizing). However, the former scenario would not be optimal because standard washing nozzles are not effective against filter cakes / scale and the latter alternative might be in some situations an over-kill leading to really expensive operations. This paper describes the use of a specialized nozzle that generates a ‘vortex’ or a fast swirling mass of fluid in the completion tubulars / open hole. Jet velocities and rotational speeds commonly exceed 460 ft/sec and 8000 rpm, respectively. Because of a specially designed internal flow guide, this nozzle entrains debris and also provides improved jet coherence and projection compared to standard jetting nozzles. This dual feature of the nozzle made it particularly attractive for use in combined sand clean out / acidizing operations of sanded up injectors. This nozzle also has no moving parts or rubber seals, seals; therefore, the commingling fluid phases no sidewould not aeffects result to the efficiency of the tool as a result of commingling fluid phases. These injectors are vertical completions with architecture ranging from perforated intervals to standalone screens. The sediments consist of a mixture of organic deposition, fine siliceous material and carbonate scales. This paper analyzes the improved post injectivity and compares results using this nozzle to those using a wash nozzle / rotating nozzle combination on the basis of cost, operational efficiency / simplicity and technical effectiveness. These results suggest that some non - aggressive scales can also be treated using this nozzle as an alternative to a rotating jetting system leading to reduction in well intervention cost through a single trip approach. Introduction and Justification Operators within the Nigerdelta as a standard have sand control measures in place. This may vary from basic standalone screens, slotted liners, pre-packed screens or standard gravel packs with wire-wrapped screens. The problem of unwanted sand production and establishing the most effective and economical means of removal during remedial treatments is a re-occurring challenge [1]. The removal of sludge and solid fill like sand, proppants, drilling, proppants, drilling solids or lost circulation materials can be circulated out of wells using a variety of techniques. Key factors governing the approach for effective removal such as completion geometry, reservoir pressure, temperature, particle size and density, etc. have been studied extensively over the years and can now be properly simulated and engineered [2]. Clean-outs within near vertical wellbores are often viewed as simple, yet best practices offer several possibilities to improve efficiency of exercise and thus save cost. The use of coiled tubing with the appropriate tools and carrier fluids that have efficient lift properties and yet low friction pressure can be a major advantage but expensive in some cases. A unique tool can improve on this process eliminating the need for complex carrier fluids, and at the same time act as an effective diversion tool for stimulation treatments. In the injector wells treated the general objective was to wash and dissolve the sediment plug across the zones of interest, clean the sandface face completion and stimulate the near wellbore region with aa unique emulsified acid system. This acid system removed the wellbore impairment caused by scale deposition/fines migration. The acid was placed by a specialized nozzle offering effective penetration of the treatment fluid via the vigorous action of high speed fluid swirl through the tool. This unique feature was successful in both fill removal and stimulation with minimum effects to the completion hardware and massive post treatment gains. The fact that the same wells were previously treated with different clean-out and stimulation techniques, we are able to closely compare the different methods and proffer the most economic, suitable and successful technique for future scenarios.
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