Acidizing Gas Wells in the Merluza Field Using an Acetici Formic Acid Mixture and Foam Pigs Abstract This paper presents the laboratory testing of acid compositions and the planning, execution, and evaluation of the acidizing operations performed in the Merluza gas field, located offshore Brazil. Since the beginning of the production, the presence of calcium carbonate scale has been observed. Laboratory experiments determined that the most efficient acid mixture to remove that scale and to clean up perforations was 7% formic acid (HFor)/ 5% acetic acid (HAc) and that the best composition for matrix acidizing was saturated di-sodium EDTA. Regular HCl compositions were not recommended due to the nature of the tubing material (13Cr) and of the high reservoir temperature. The use of foam pigs to keep the acid at the desired position and to prevent the reaction products from decanting to the formation, and the application of the organic acid mixture to clean up the perforations were very successful, increasing the gas production rate by 740.000 m3/day. Because of this, the matrix acidizing with EDTA was postponed. These results show that Hac/H for compositions are a viable solution to acidize wells where HCl cannot be used, such as HPHT wells, or where production equipment contain chromium. P. 67
Scale deposition in producing well bores is becoming a serious problem to the oil industry. In situations of injected seawater breakthrough, the problem is especially hard, since the growth is often Barium or Strontium Sulfate, both of which almost completely insoluble. The initial scale growth will change the surface roughness in the production tubing, raising the frictional pressure drop and reducing the production rate. Further growth will decrease the flowing area and hinder the production. Ultimately the scale can restrict the production tubing sufficiently to prevent access for tools into the lower sections of the well and finally it can bridge over completely. The range of options for scale removal goes from a simple brush run using slick line or basic chemical treatment up to a full rig workover to replace the production string. Very often through-tubing treatments using coiled tubing are used due to the savings compared to the rig cost. This technology can convey tools for mechanical scale removal and also works as a fluid conduit for chemical treatments. The objective of this paper is to describe the coiled tubing abrasive jet technology used to successfully clean, for the first time, heavy affected Barium Sulfate scale in production tubing in Brazilian offshore operations and its impact on the well economics. Introduction The water quality constant variation throughout the well life, in the majority of the cases, is one of the main factors for the inorganic scale formation, especially where the driver aquifer is maintained by sea water injection. Normally on those situations, the scale builds up in the perforations and/or the production string, which in both cases reduces the flow area and/or induces completion components failures as gas-lift or sub-surface safety valves (see Fig. 1). The production history shows that, in the case of injected seawater breakthrough, the resulted scale is normally Barium or Strontium Sulfate, which has very limited solubility. Also the working experience with those scales points out that, in case of thick deposits the conventional removing methods (Scale Dissolver, Brushes and Down Hole Motor) are completely inefficient or not economically feasible. Because of those challenges, normally the only found solution for this kind of scale problems is the completion tubing and hardware substitution which is normally very costly and in some cases very difficult due to logistic limitations. The creation and development of a new through-tubing technology based on high velocity abrasive jetting system has completely changed the standard approach to remove hard scale inside tubing and hardware. This technology has already been applied in several locations around the world. This paper describes the use of this jetting technique for the first time in Brazil and outlines the different steps to carry out the design, the execution and evaluate the results of a successful operation. Well History and Background Petrobras has a large history of hard scale problems (Barium Sulfate) in several of their mature offshore fields. The well in question was located in a fixed offshore production platform, which did not have a working rig available and because of that, the standard approach of tubing replacement, in case of thick hard scale deposits, was not possible. Historically several treatment alternatives were tried in similar wells in this field. Sometimes, these treatments have successfully removed the scale from the tubing wall. However, due to debris size and gravity, they decanted to the bottom of the hole, even shutting the perforations off and decreasing the well flow. So, the problem was enhanced, because it was necessary a workover, not only to change the string, but further, to cleaning the bottom hole. Because of the well production declining, that was affecting the whole field target production, on May 2003 it was performed a slick line calibration, and it was found a considerable reduction in the Tubing ID from 2.992" to 1.83" in the estimated interval from 2556 to 3027 meters. Due to those results and based on the available options, it was decided to attempt the scale removal using the coiled tubing abrasive jetting technology.
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