Tar mats at the oil-water contact (OWC tar mats) in oilfield reservoirs can have enormous, pernicious effects on production due to possibly preventing of any natural water drive and precluding any effectiveness of water injectors into aquifers. In spite of this potentially huge impact, tar mat formation is only now being resolved and integrated within advanced asphaltene science. Herein, we describe a very different type of tar mat which we refer to as a "rapid-destabilization tar mat??; it is the asphaltenes that undergo rapid destabilization. To our knowledge, this is the first paper to describe such rapid-destabilization tar mats at least in this context. Rapid-destabilization tar mats can be formed at the crest of the reservoir, generally not at the OWC and can introduce their own set of problems in production. Most importantly, rapid-destabilization tar mats can be porous and permeable, unlike the OWC tar mats. The rapid-destabilization tar mat can undergo plastic flow under standard production conditions rather unlike the OWC tar mat. As its name implies, the rapid-destabilization tar mat can form in very young reservoirs in which thermodynamic disequilibrium in the oil column prevails, while the OWC tar mats generally take longer (geologic) time to form and are often associated with thermodynamically equilibrated oil columns. Here, we describe extensive data sets on rapid-destabilization tar mats in two adjacent reservoirs. The surprising properties of these rapid-destabilization tar mats are redundantly confirmed in many different ways. All components of the processes forming rapid-destabilization tar mats are shown to be consistent with powerful new developments in asphaltene science, specifically with the development of the first equation of state for asphaltene gradients, the Flory-Huggins-Zuo Equation, which has been enabled by the resolution of asphaltene nanostructures in crude oil codified in the Yen-Mullins Model. Rapid-destabilization tar mats represent one extreme while the OWC tar mats represent the polar opposite extreme. In the future, occurrences of tar in reservoirs can be better understood within the context of these two end members tar mats. In addition, two reservoirs in the same minibasin show the same behavior. This important observation allows fluid analysis in wells in one reservoir to indicate likely issues in other reservoirs in the same basin.
A Jurassic oil field in Saudi Arabia is characterized by black oil in the crest, with heavy oil underneath and all underlain by a tar mat at the oil-water contact (OWC). The viscosities in the black oil section of the column are similar throughout the field and are quite manageable from a production standpoint. In contrast, the mobile heavy oil section of the column contains a large, continuous increase in asphaltene content with increasing depth extending to the tar mat. Both the excessive viscosity of the heavy oil and the existence of the tar mat represent major, distinct challenges in oil production. A simple new formalism, the Flory-Huggins-Zuo (FHZ) Equation of State (EoS) incorporating the Yen-Mullins model of asphaltene nanoscience, is shown to account for the asphaltene content variation in the mobile heavy oil section. Detailed analysis of the tar mat shows significant nonmonotonic content of asphaltenes with depth, differing from that of the heavy oil. While the general concept of asphaltene gravitational accumulation to form the tar mat does apply, other complexities preclude simple monotonic behavior. Indeed, within small vertical distances (5 ft) the asphaltene content can decrease by 20% absolute with depth. These complexities likely involve a phase transition when the asphaltene concentration exceeds 35%. Traditional thermodynamic models of heavy oils and asphaltene gradients are known to fail dramatically. Many have ascribed this failure to some sort of chemical variation of asphaltenes with depth; the idea being that if the models fail it must be due to the asphaltenes. Our new simple formalism shows that thermodynamic modeling of heavy oil and asphaltene gradients can be successful. Our simple model demands that the asphaltenes are the same, top to bottom. The analysis of the sulfur chemistry of these asphaltenes by X-ray spectroscopy at the synchrotron at the Argonne National Laboratory shows that there is almost no variation of the sulfur through the hydrocarbon column. Sulfur is one of the most sensitive elements in asphaltenes to demark variation. Likewise, saturates, araomatics, resins and asphaltenes (SARA); measurements also support the application of this new asphaltene formalism. Consequently, the asphaltenes are very similar, and our new FHZ EoS with the Yen-Mullins formalism properly accounts for heavy oil and asphaltene gradients.
Typical shale well completions involve massive, multistage fracturing in horizontal wells. Aggressive trajectories (with up to 20°/100 ft doglegs), multistage high-rate fracturing (up to 20 stages, 100 bbl/min), and increasing temperature and pressure of shale reservoirs result in large thermal and bending stresses that are critical in the design of production casing. In addition, when cement voids are present and the production casing is not restrained during fracturing, thermal effects can result in magnified load conditions. The resulting loads can be well in excess of those deemed allowable by regular casing design techniques. These loads are often ignored in standard well design, exposing casing to the risk of failure during multistage fracturing. In this work, the major factors influencing normal and special loads on production casing in shale wells are discussed. A method for optimization of shale well production casing design is then introduced. The constraints on the applicability of different design options are discussed. Load-magnification effects of cement voids are described, and a method for their evaluation is developed. Thermal effects during cooling are shown to create both bending stress magnification and annular pressure reduction caused by fluid contraction in trapped cement voids. This can result in significant loads and new modes of failure that must be considered in design. The performance of connections under these loads is also discussed. Examples are provided to illustrate the key concepts described. Finally, acceptable design options for shale well production casing are discussed. The results presented here are expected to improve the reliability of shale well designs. They provide operators with insight into load effects that must be onsidered in the design of production casing for such wells. By understanding the causes and magnitude of load-augmentation effects, operators can manage their design and practices to ensure well integrity.
The cold-production-recovery process, also known as cold heavy-oil production with sand (CHOPS), is a method for enhancing primary heavy-oil production by aggressively producing sand. It is successful in vertical (or slanted or deviated) wells in western Canada. In this process, large amounts of sand are produced on a continuing basis along with heavy oil. Attempts at cold production in horizontal wells have not been particularly successful. When sand production has been generated in horizontal wells, these wells have tended to become plugged with sand.
This paper presents the results of experiments performed to assess the feasibility of applying cold heavy-oil production in horizontal wells that have been completed with slotted liners using less-aggressive (i.e., managed) sand-production strategies. Specifically, the effects of slot size, confining stress, fluid velocity, and sand-grain sorting on sand production were investigated.
The results indicate that slot-size selection is critical for establishing "sand on demand." From the experiments, a correlation between slot size and controlled sand production was found for well-sorted sands. This correlation should allow for the specification of appropriate slot sizes for target reservoirs containing well-sorted sands.
In the experiments, when flow rates resulted in low but persistent sand production, channels and/or elliptical dilated zones were created that greatly enhanced the effective permeability near the slot. This observation suggests that producing at low and steady sand cuts for a long period of time might bring two benefits: a way to transport the sand out of the well without causing plugging and the creation of high-permeability channels or zones that can improve production from the reservoir.
To summarize, if the appropriate slot size were combined with the right drawdown rates, controlled sand production could be achieved, with attendant significant increases in permeability. This suggests that substantially increased oil-production rates could be achieved from horizontal wells if sand-production rates could be maintained at low but persistent levels.
One of the major challenges of drilling and completion of oil and gas wells is the uncertainty in the formation fracture gradient and the fracture pressure. It is not uncommon that many drilling companies have spent money, resources and time in drilling and completing wells that should have been simply and optimally done. Fracture gradient evaluation constitutes one of the essential parameters in the pre-design stage of drilling operations, reservoir exploitations and stimulations. Several calculation methods and computer models have been presented in the literature for different regions of the world. Most of these techniques were based on either parametric or empirical correlations, which required a prior knowledge of the functional forms or the use of empirical charts which were not very accurate.
This paper presents an innovative method of predicting formation fracture gradient for Gulf of Guinea region. A combination of "Mathew and Kelly?? correlation, "Hubbert and Willis?? correlation and Ben Eaton mathematical models were used in developing the simplified technique based on field data from the Gulf of Guinea. The model compared favorably with the existing fracture gradient results in the Gulf of Guinea with less than 1 % deviation from other correlations thereby saving the rigors and time in using tables, charts and other long techniques. Although the method was developed specifically for the Gulf of Guinea, it should be reliable for other similar areas provided that the variables reflect the conditions in the specific area being considered.
A Jurrasic oilfield in Saudi Arabia is characterized by black oil in the crest and with mobile heavy oil underneath and all underlain by a tar mat at the oil-water contact. The viscosities in the black oil section of the column are fairly similar and are quite manageable from a production standpoint. In contrast, the mobile heavy oil section of the column contains a large continuous increase in asphaltene content with increasing depth extending to the tar mat. The tar shows very high asphaltene content but not monotonically increasing with depth. Because viscosity depends exponentially on asphaltene content in these oils, the observed viscosity varies from several to ~ 1000 centipoise in the mobile heavy oil and increases to far greater viscosities in the tar mat. Both the excessive viscosity of the heavy oil and the existence of the tar mat represent major, distinct challenges in oil production. Conventional PVT modeling of this oil column grossly fails to account for these observations. Indeed, the very large height in this oil column represents a stringent challenge for any corresponding fluid model. A simple new formalism to characterize the asphaltene nanoscience in crude oils, the Yen-Mullins model, has enabled the industry's first predictive equation of state (EoS) for asphaltene gradients, the Flory-Huggins-Zuo (FHZ) EoS. For low GOR oils such as those in this field, the FHZ EoS reduces to the simple gravity term. Robust application of the FHZ EoS employing the Yen-Mullins model accounts for the major property variations in the oil column and by extension the tar mat as well. Moreover, as these crude oils are largely equilibrated throughout the field, reservoir connectivity is indicated in this field. This novel asphaltene science is dramatically improving understanding of important constraints on oil production in oil reservoirs.
Wellbore strengthening techniques have been used in recent years to increase the capability of wellbores to maintain higher pressures. By increasing the fracture resistance of formations, operators can save rig-time and large volumes of drilling fluids.
The Luna-41 well, offshore Italy, intersects a critical interval comprising high pressurized formations overlaying a lower pressure depleted zone. The initial plan for the well was to divide this interval into two separate hole sections using two different mud systems. A casing string would have been set to isolate the shallower high pressure region followed by an expandable liner to isolate the over pressured shales laying above the depleted reservoir level.
An alternative design was proposed that required only one fluid system and a single casing string, thus saving an expandable liner. Thanks to the wellbore strengthening application and the proprietary continuous mud circulation device, the accomplished well program allowed an 8-day rig-time reduction and a 3-MMUSD cost saving.
A specific modelling tool developed for wellbore strengthening applications was used to assist with fluid design. The tool calculates the width of microfractures induced by differential pressure and the Particle Size Distribution (PSD) of carbonate materials required to plug such microfractures and ultimately strengthen the wellbore.
The mud formulation for Luna-41 was tested in the laboratory using a Pore Plugging Apparatus (PPA) and aloxite discs with pore sizes corresponding to the calculated microfracture width. The fluid used to drill the critical interval was a salt saturated system based on polyglycerol complex and supplemented with a polyamine inhibitor.
The field application was a success. The depleted zone was drilled without incurring lost circulation. This paper describes the results of the field application as well as the fluid engineering process and laboratory testing to highlight the benefits - such as accessing depleted reservoirs and saving casing strings - that wellbore strengthening combined with a continuous mud circulation system can bring to the industry.
Pore pressure is a key parameter in controlling the well in terms of reservoir fluid pressure. An accurate estimation of pore pressure yields to better mud weight proposition and pressure balance in the bore hole. Current well known methods of pore pressure prediction are mainly based on the differences between the recorded amount and normal trend in sonic wave velocity, formation resistivity factor (FRF), or d-exponent (a function of drilling parameters) in overpressured zone. The majority of the techniques are based on the compaction of specific formation type which need localization or calibration. They occasionally fail to proper response in carbonate reservoirs.
In this research, a new method for calculating the pore pressure has been obtained using the compressibility attribute of reservoir rock. In the case of overpressure generation by undercompaction (which is the case in most of the reservoirs), pore pressure is depended on the changes in pore space which is a function of rock and pore compressibility. In a simple way, pore space decreases while the formation undergoes compaction and this imposes pressure on the fluid which fills the pores. Generally, rock compressibility has minor changes over a specific formation, but even this small amount must be considered. Thus, the statistical tools should be used to distribute the compressibility over the formation. Therefore, based on the bulk and pore compressibility achieved from the special core analysis (SCAL) or well logs in one well, the pore pressure in the other locations of a formation could be predicted.