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 growing interest in deeply buried reservoirs increasingly leads operators into extreme drilling conditions characterized by high hydrostatic pressure (over 20.000 psi) and high temperatures (over 300 °F), combined with hard formations (high CCS-40,000+ psi). Drilling in such environments requires specific adaptations of well design and in equipment selection. Additionally, these wells present multiple issues in well control, drilling and completion operations, and make the entire operation technically more complex and financially more risky.Referencing the specific example of drilling operations, the great depths and increased formation compaction leads to very low rates of penetration (ROP) and considerably extended operation time. These low ROPs equate to low depth of cut, and as a direct consequence the typical size of the drilled cuttings recovered at the shakers is extremely low (0.020 to 0.20 mm). This makes the cuttings irrelevant for the purpose of geological identification of the formations. This situation occurs with both impregnated diamond and PDC drill bits in these applications at low ROPs. If the downhole conditions (HPHT) exceed the MWD or LWD specifications, accessing critical geological information will necessarily require a challenging coring job, dramatically increasing the operation time and thus the cost of the well.
The drilling of hard formations usually requires specific drilling systems to achieve an efficient rate of penetration. Due to the limited depth of cut, the solution often involves the use of high RPM drive systems such as turbines or high speed motors and a fixed cutter product is usually the drill bit of choice. With increasing well depth the roller cone option is normally avoided due to the low penetration rates and limited rotating hours obtainable with these drill bits. Roller cone use invariably leads to multiple trips and a significant increase in the time and cost of the drilling operation, especially for deeply buried reservoirs where hard rocks are generally encountered and bit diameter is reduced at TD. Unavoidably, in some applications roller cone bits continue to be used in order to obtain the quality cuttings necessary for the geologist to correctly assess the formation. This is typically the case when the reservoir interval is encountered and exact formation knowledge is critical. A new drilling solution has been developed to address this situation and is presented in this paper. The idea consists of a fixed cutter bit which leaves the center of the hole uncut. The lack of bit center leads to the creation of a core. This core is broken by the bit itself and ejected at the side through a specifically designed junk slot. The core is then carried to the surface along with the other cuttings. This process leads to very high quality cuttings for surface examination. The improvement is especially important where normal cuttings quality is poor due to the use of turbines or high speed motors together with impregnated diamond bits. Importantly, as the center part of the bit is uncut this new technology also provides a significant increase in the rate of penetration. In a fixed cutter bit the center is one of the most inefficient cutting areas as the rotational speed of the cutter is low to null. By removing the center, an increased ROP efficiency in the range of 15 to 25 percent can be obtained depending upon the bit diameter. The field tests that are presented in the paper show quantifiable improvements in terms of geological sample quality, cutting efficiency and drilling cost reduction.
Deeply buried reservoirs are one of the main options to extend the oil & gas reserves and to keep on satisfying the growing need of energy. In this context, the ability to determine the temperature profile of the mud while drilling is a crucial issue from the design of a well to the control of the rheological properties of the mud. The present work is a study of the heat transfers occurring between the mud and the formation while circulating in a well. Firstly, a model, based on an empirical Nusselt law, was built and implemented in a numerical software. Secondly, we compared the results found with the numerical software with field data, which was made possible thanks to the collaboration of Total. Introduction The estimation of the "peak oil" relies on the knowledge of the identified and recoverable oil & gas reserves. Currently, the producing countries still estimate their reserve to contain enough oil & gas for another 40 years of production. Several ways exist to keep this estimation and postpone the "peak oil", among which is the production of deeply buried reservoirs. This is a challenge the producing oil companies will have to take up. Nevertheless, the deeper the drilling, the bigger the pressure and thermal constraints are. High pressure and high temperature mandate the development of new technologies such as drilling fluid which is able to keep its rheological properties. The mud has to maintain its efficiency in:–removing the cuttings from the well–stabilizing the borehole to prevent any collapse of the well–cooling down, cleaning and lubricating the bit–controlling the fluid loss–suspending the cuttings in case of a pump stoppage In this context, it is important to be able to assess the temperature of the drilling mud and to make predictive calculations of the mud temperature while drilling. This information would be priceless for the mud engineer, mud logger, drilling engineer, etc., to help to choose the right composition of mud at the right time (E. Gao et al. 1998 and B. Corre et al. 1984), especially when the temperature reaches the threshold when MWD tools do not work anymore. That's the reason why we worked on building a model which correctly represents the heat exchange phenomena between the mud and its environment. Ultimately, the goal is to be able to accurately predict the temperature profile of the mud all along the well with potential applications in real time monitoring or well design. State of the Art Showing the level of interest in the topic, there have been a number of papers which have been written about the mud temperature when circulating in a well.
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