Experiments were conducted to evaluate the rate of water vaporisation and the consequent permeability reduction caused by the flow of dry gas through porous media. Two sets of porous media were studied: unconsolidated Ottawa sandpacks and consolidated Berea cores, as well as various salinity brines ranging from 0 to 150 g/litre of NaCl. The experiments were conducted at an initial water saturation of about 14% for sandpacks and 24.6% for Berea cores. Tests indicate that the rate of water vaporisation increases with gas flow rate and decreases with salinity. The vaporisation of water from the porous media can result in halite drop-out. This might cause a reduction in the permeability of the porous media. The experiments showed that the reduction in permeability for sandpacks ranged from 0% for the lowest salinity to around 21% for the highest salinity used. In consolidated Berea cores this reduction ranged from 9% to 53% for the lowest and highest salinity respectively. Introduction The flow of dry gas through porous media vaporises water in order to fulfil the thermodynamic requirements at a given pressure, temperature and salinity of the brine saturating the rock. Also, the development of an increasing number of high- pressure, high-temperature (HT/HP) fields has arisen important issues concerning inorganic deposition in these systems given their frequently high salinity brines. As the pressure declines at constant temperature, water is vaporised given the increase of the molar water content in the gaseous phase. This produces an over-concentration of dissolved salts and the solubility limit can be reached with the subsequent salt precipitation. Several authors have reported field cases where precipitation of NaCl is believed to be the cause of formation damage and it has been usually associated to water vaporisation1,2,3,4. Background Dodson and Standing5 reported experimental studies in PVT cells to determine the solubility of a natural gas (?=0,655) in brine and to determine the solubility of water vapor in natural gas. The experiments were performed at pressures ranging from 500 to 5000 psia, temperatures from 100 to 250°F and brines having up to 25000 ppm of total solids. It was found that the amount of water in the gas phase increases with temperature and decreases with pressure and solids content. Place and Smith1 reported a severe decline in productivity for the well Shell Ridgway 1-R in the Southwest Pineywoods Field, MS. After observation of the decrease in Cl-/K+ and Cl-/Li+ ratios in the produced fluids, the authors concluded that the deposition of NaCl at the open hole was responsible for the significant loss of productivity. Even though the water saturation in the main pays was considered to be inmmobile (15% to 26%), the overlying and isolated zones had water saturations in the range of 35% to 50%. Under this scenario, the flowing gas would evaporate water from the main pays increasing the concentration of the brine. Also, if less concentrated brine is flowing into the wellbore from the high saturation zones, the gas would evaporate this less concentrated brine with the subsequent NaCl deposition. The author's point out that only when the salinity of the pay zones exceeds that of the overlying high saturation zones the episode of salt deposition and plugging will be initiated. Morin and Montel3 studied the dehydration process in order to predict the conditions for precipitation of NaCl in the tubing. The authors mention three situations that contribute to the vaporisation process including the flow of gas from layers at different water saturation, important pressure drop with small temperature decrease and the increase in vaporisation when the gas flows from the near wellbore area to the well. The latter situation appears because the porous media causes a decrease in the water content of gas compared to the value outside of it given that capillarity and adsorption tend to retain the water in the rock. The authors remark the influence of water saturation on the rate of water vaporisation.

In the design of oil and gas well barriers, the quality of the cemented casing must be assessed to optimize the cost of the operation. Borehole cement evaluation logs are used to verify the quality of the cement bond to the casing and formation and identify any defects that could compromise the quality of the annular seal. This study used ultrasonic logging to evaluate reference barrier cells constructed with known defects. In this joint project between industry representatives and a research institute, reference barrier cells were created to simulate downhole oil well conditions, such as free/cemented pipe, sagged barite, gas environment, mud channel, microannulus, axial hole, and cemented control line. The cell concept was developed with an emphasis on low cost and ease of use. Different cell configurations could be quickly coupled to provide the required tool lengths and measurement points for logging experiments. The tubing and annulus pressures (maximum 150 bar) and fluids can be varied according to the test requirement. Each reference cell was constructed to highlight a condition that is typically encountered in an oil well. Full-scale logging was carried out in which multiple reference cells were coupled vertically on a drilling rig. Data were acquired at multiple resolutions, while applying pressure (0, 100, and 150 bar) to the outer annulus, thereby varying microannulus size. Data from the pulse echo and flexural sensors provided a detailed image highlighting the condition of the bond behind the inner casing. Laboratory experiments were performed to characterize the reference cells, including measurements of strain and infrared thermography to visualize the leakage paths. The log response was found to be consistent with the physical observation and laboratory experiments. During the cement evaluation of a tubular casing, interpretation is made based on log response under certain well conditions. Inferences are based on fundamental sensor physics, laboratory experiments, common knowledge, and contemporary understanding of the technology. Comparison of ultrasonic tool response against reference barrier cells validates the measurements, which are crucial in the decision-making process during well construction and abandonment. The paper describes the full-scale test performed at the research institute, including the laboratory experiments and log data acquired.

In an oilfield well, when the annulus contents behind the casing are evaluated using ultrasonic measurements, the properties of the borehole mud, such as acoustic impedance and fluid velocity, are critical input for the accurate determination of acoustic impedance of annulus material and its subsequent bond quality. In deviated or horizontal wells, mud settling, and subsequent segregation leads to azimuthal and depth uncertainties in annulus evaluation. Typically, due to gravity, mud segregates, with the light component at the top and heavier component at the lower side of the well. In a non-homogeneous mud, using a single mud impedance value for computing acoustic impedance of the annulus can lead to ambiguous answers with uncertainties. Traditionally, it has been a challenge to accurately measure and apply these variations in acoustic impedance of the mud to precisely interpret the bond quality in the annulus. A novel pulse-echo processing scheme called R+ inversion, based on a 3-parameter inversion approach, eliminates, to a great extent, the dependence on prior knowledge of the borehole mud. The 3-parameter inversion can also reveal conditions such as mud deposition and segregation in deviated pipes. This new processing enables easier and accurate interpretation of the annular content together with essential information about the logging fluid. Four case studies established the successful implementation of R+ inversion in deviated wells in the Norwegian Continental Shelf (NCS) with azimuthal uncertainties in the mud acoustic impedance to provide reliable annulus interpretation. These measurements correlate and are validated using sonic logs as well as flexural attenuation measurements, thus providing confidence in the results and decisions. The case studies compare acoustic impedance results using legacy processing and R+ inversion processing. The limitation to use the azimuthal variations of mud in the traditional processing sometimes leave unanswered questions related to the bond quality affecting the intervention decisions expected from the bond log. With the help of R+ inversion, the operator managed to take informed intervention decisions faster, thus saving rig time and cost. Four case studies are explained in the details that demonstrate and validate the importance of R+ inversion when borehole mud settling occurs azimuthally, thus overcoming previous limitations of mud impedance computation and usage. Cement evaluation using R+ inversion enables accurate and critical decision making during new well construction, intervention, plugging, and abandonment in all conditions, irrespective of the casing sizes and cement types.

The Gyda field in the North Sea operated by Repsol was proven in 1980 and the platform started producing in 1990. In June 2017, the Norwegian authorities approved the decommissioning plan for the Gyda field. The decommissioning scope included the permanent plugging of 32 wells in the field. Decommissioning is estimated to cost several hundred million dollars and is expected to finish in 2022. As per the NORSOK standards, each well needs to have confirmed barriers to isolate inflow zones, both for preventing from flowing to the surface and hindering crossflow between them. Cement and creeping formation are both considered to be potentially effective barrier elements. However, the criteria and verification methods used to confirm formation creep and cement as barrier elements are different and hence require an innovative interpretation technique which is presented in this paper. As per the regulations and standards, it is critical not only to evaluate the quality of the circumferential bond for cement and formation creep but also to determine their respective bond length. The most important measurement to accurately determine those criteria in each well is through the ultrasonic and flexural attenuation tool. However, interpretation to differentiate formation creep from cement presents challenges, especially when they have similar ultrasonic properties. Quite often, they coexist at the same depths on different sides behind the casing. Barrier evaluation becomes even more challenging with added complexities such as borehole mud settling due to high deviation, high eccentricity, casing damage, or presence of a microannulus. This paper discusses the techniques and interpretation methods used to accurately evaluate barrier elements, differentiate between cement and formation creep, estimate the tops of cemented areas, and eliminate complex challenges posed by mud, deviation, eccentricity, and wet microannulus sections. Successful and accurate determination of the potential presence and location of annulus barrier elements has been fundamentally important for Repsol to meet the regulatory requirements. A special interpretation technique was established using integrated data evaluation to differentiate creeping formation from cement. This technique successfully determined accurate barrier intervals, helping to meet all the regulatory requirements. The processes and methods have been audited and evaluated by the Petroleum Safety Authority Norway.

This paper is based on the analysis of the ultrasonic/sonic data of the 9 5/8-in. casing logging of the 14 wells of the Varg field within the Norwegian Continental Shelf. While writing this papper Varg field was undergoing a plug and abandonment (P&A) phase after 19 years of production. High-quality bonding is observed behind the 9 5/8-in. casing far above expected theoretical top of cement within single casing areas. This bonding is attributed to the formation influence. Formation is used as an alternative to traditional cement barriers during P&A, and its use is regulated by the legislation. The paper aims to develop better understanding of the mechanisms responsible for formation bonding development. The percentage of observed bonding at "high" and "high and moderate-to-high" quality is calculated within each well and is related to the various factors that could influence formation bonding development. Factors such as duration of time lapsed from well completion to well logging, type of well (producer versus injector), geological formation, type of drilling mud used, duration of production periods, volumes of production, and well deviation and azimuth were looked at to determine observable trends and relationships. The results of the study allowed us to conclude which factors are critical or influence formation bonding. Based on the observations, recommendations can be made for the selection of the first well to be logged on the planned P&A campaigns. Correct selection of the first well saves time and resources on the formation testing for the qualification of the formation as a barrier.