Scale inhibitors (SI) have been applied very successfully over many years in oilfields to prevent the formation of mineral scale. Both barium sulphate and calcium carbonate scales may be prevented using inhibitors, although in this work we will focus on the more difficult barite inhibition problem. A number of publications have appeared discussing the mechanisms by which barium sulphate scale inhibitors operate to prevent or retard scale formation. The mechanisms are discussed here in terms of (a) nucleation inhibition where the scale proto-crystals forms but are then disrupted or redissolved by the action of the inhibitor molecules, and (b) crystal growth inhibition where the inhibitor is thought to adsorb or interact with the active crystal growth sites (growing edges or spirals) hence retarding or stopping the crystal growth process. Both of these mechanisms are consistent with the inhibition of mineral scale at "threshold" levels, typically for MIC=0.5 - 20 ppm (MIC=minimum inhibitor concentration for a defined level of inhibition for a given test procedure). The MIC is always considerably below stoichiometric values in terms of the scale inhibitor to mineral scale molar ratios. It is known that most inhibitor types from the small molecular phosphonates (e.g. DETPMP) to polymeric species (e.g. PAA, PVS, PPCA) actually operate through both of the above mechanisms although one of these may predominate for specific species. Previous work has established that, broadly speaking, smaller phosphonates operate principally as crystal growth inhibitors and polymeric species work mainly as nucleation inhibitors. A number of factors are known to affect the inhibition efficiency (IE) of scale inhibitors against barite formation, the main ones of concern here being pH, temperature and the calcium and magnesium levels in the scaling brine mixture. The general observed effects of these factors have been described in the literature and will be discussed in detail in this paper. However, no complete description of the mechanism of barite inhibition has appeared which clearly and consistently explains all of the observed effects of these parameters for different scale inhibitor types. It is the central aim of this paper to present a complete and consistent set of mechanisms for barite inhibition which may vary in degree for different inhibitor types. Our proposed mechanisms are based on a wide range of observations from the open literature analysed with our own experimental and modelling results. Experimental Details To develop a set of mechanisms for barite inhibition, we use a number of experimental techniques and data sources. Since the experimental techniques used are well known, we refer the reader to literature descriptions for details. Static inhibition (bottle) tests and dynamic (tube blocking) tests are both very well known techniques for determining inhibition efficiency (IE) and are described in detail elsewhere1,2. The brine compositions used in the new static IE results presented here are given in Table 1. The scale inhibitors used in this study are: diethylene triamine penta (methylene phosphonic acid) (DETPMP), phosphino polycarboxylic acid (PPCA) and polyvinyl sulphonate (PVS) and their structures are given in Fig.1 Crystal structure measurements of the a-axis deformation of the barite lattice from this work and the literature are used here3,4. The theory behind this is given in Ref. 5. We also present some simple calculations of the equilibrium system containing calcium ions, magnesium ions and scale inhibitor. Results are given for the Ca/Mg/DETPMP system close to MIC levels for which the stability constants are known6.

No abstract

No abstract

Inorganic precipitates (scales) are one of the major flow assurance concerns in offshore oil and gas production, and lead to significant reductions in productivity and costly workovers if allowed to form uncontrolled. Scale prevention by the use of chemical inhibitors, applied either by continual injection or by squeeze treatment into the near wellbore formation, has generally been regarded as the most cost effective solution to the problem. The increased development of subsea satellite production brings with it a number of additional challenges to the chemical control of inorganic scale. The requirement to control scale at low seabed temperatures (4 - 5°C) over long residence times presents a particular challenge. This paper examines the performance of three generically different scale inhibitor species including a polyvinyl sulphonate (PVS), a phosphino polycarboxylate (PPCA) and a penta phosphonate (DETPMP) under typical North Sea Scaling conditions. The relative performance of the different inhibitor species against Barium Sulphate scale is compared at temperatures between 5°C and 5°C and up to 22 hours residence times. Results show that the performance of different inhibitors are significantly affected and to different degrees as a function of temperature, and that this is not simply related to changes in supersaturation against barium sulphate precipitation. The performance of the phosphonate species DETPMP is significantly reduced at lower temperatures and this is related to reduced adsorption at the barium sulphate surface. However the performance of the PVS is significantly improved, indicating that such species offer the potential for improved scale control at lower seabed temperatures. PPCA is only marginally affected by reduced temperatures in a manner more consistent with the increased supersaturation against barium sulphate. Introduction and Background As the search for new hydrocarbon reserves and the need to maximize recovery from mature fields continues, many operators are moving away from conventional platform developments in favour of subsea wells. In these situations, wellheads are frequently located subsea, with long seabed flow lines connecting the well to production facilities such as an FPSO or an existing platform some distance away. Such installations are invariably complex, with many satellite wells tied back to a single central facility, where fluids from several wells may be mingled in a shared flow line, making access to individual wells for workovers, squeeze treatments or fluid analysis extremely difficult and prohibitively expensive, especially in deepwater development systems as have been described previously.1–6 The increased development of subsea satellite production, coupled with the extremely high intervention costs associated with such wells, brings with it a number of additional challenges to the chemical control of inorganic scale. Water is most dense at the seabed, and will usually be in the range 3.3°C - 6.1°C (38°F - 43°F), so the produced fluids in non-insulated lines experience significant cooling before arriving at the production facilities. The extreme length of subsea flow lines particularly in deepwater systems (the longest reported to date is 60 miles in the Mensa field operated by Shell in the Gulf of Mexico3) increases residence times to up to twenty four hours, compared to the minutes or hours associated with conventional platform developments. These low temperatures and long residence times combine to make a formidable challenge for scale inhibition. The supersaturation of a given solution of barium sulphate increases at lower temperatures due to the reduced solubility of the mineral. In addition, the performance of certain chemical inhibitors at low temperatures has raised some concern. In particular, different workers have demonstrated that the performance of a conventional phosphonate DETPMP (di-ethylene tri-ammine penta-methylene phosphonic acid) can be significantly reduced at lower test temperatures with certain polymers being effected affected much less significantly.5,7,8

A study was performed in the shallow waters of the MacKenzie Delta area near Tuktoyaktuk, N.W.T., Canada, to evaluate equipment, test procedures, and techniques using a seismic cone penetrometer and operating on the landfast ice in winter. Seismic cone penetration testing was performed to determine the compressional and shear wave velocities of the subsurface sediments using a downhole technique. Several seismic sources and receivers were tested to evaluate their effectiveness. Typical results are presented and briefly discussed. Key words: downhole, seismic, P-wave, S-wave, velocity, in situ, measurement, shallow offshore, cone penetration test.