An extensive research programme was launched with the aim at evaluating the potential of seven different polyamino carboxylic acids (PACAs) using both technical and economic aspects. Dissolving capacity of each compounds was determined under equilibrium conditions as a function of concentration for barite and reservoir rock (sandstone). The absolute and relative selectivity of dissolution, viz. the matrix effect of formation rock were precisely analyzed. Dissolution of barium sulfate, calcium and magnesium carbonate and iron compounds was followed in time, and thus, the kinetic calculation could also be made.
Results of the research programme provided reliable data for comparison of dissolution capacities under identical chemical (equilibrium, kinetic, dissociation, etc.) conditions. On the basis of the experimental findings it was concluded that the sequence of dissolution capacity is different if the results were evaluated in technical or economic senses. It was also shown that the preferentially used compounds (e.g. DTPA, and particularly EDTA) are not the best choices when technical considerations are only enforced. The economic grounds, however, may partly justify their application at the expense of poor efficiency. The systematic analysis of the mentioned dissolvers may open new vistas in mitigation of formation damage caused by barite or barium sulfate scales and accelerate the search for more effective mixtures of complex-forming agents to be applied under field conditions.
Introduction
Deposition of barium sulfate in surface facilities at the Saratoga field (Texas) has been recognized nearly a hundred years ago. In the middle of the thirties it was also realized that precipitation of scales may take place not only in the bottom hole or the production tubing, but also in the vicinity of wells causing the well known formation damage. In the same time substantial amount of knowledge in physical chemistry has been accumulated which gave satisfactory explanation for solid phase formation from homogeneous solutions. The supersaturation, seed formation and crystal growth as subsequent steps of scale formation are still regarded as main elements of the unfavorable process. In the mean time detailed analysis of factors influencing the side reactions and part processes contributed significantly to better understanding of mechanisms, but the information obtained opened also new vistas in predicting and controlling the precipitation of different materials from formation water.
In practice of the hydrocarbon production the "scale" or "injectivity/producibility" problems have always been omnipresent and hence, a great deal of efforts had been made in the past decades to prevent scale formation or to develop efficient technology to remove the precipitated solids if they already formed. Therefore it is not a surprising remark, as Sloat1 stated as early as 1963, that the scale inhibitors are the real "work horses" of the industry and without organic phosphates type well packing a stable production rate can not be maintained at most of the oil fields. Although the application of scale inhibitors has started much earlier in karstic formations2,3, blocking of seeding or crystal growth by these compounds remained effective for different types of scales until now. On the other hand, it is also evident that acidization serves the same important purposes in alleviation of formation damage if acid soluble compounds, usually inorganic scales are responsible for deterioration of injectivity or producibility in wells. These two techniques, inhibition or dissolution of scales, still represent fundamental technologies in oilfield service and they are the most frequently applied stimulation techniques in both continental and off-shore fields.
The formation damage may ensue already during drilling. Recently, extension of exploration and drilling activity towards greater depth, and as a result, application of weighing additives more widely, increase logically the danger of formation damage. It is true that partial replacement of barite with acid soluble materials may offer a temporary solution, but on the other hand, the colloidal barite having numerous advantages over other mud components, might result worsening situation.
