Recently two multilateral horizontal wells have been completed offshore using dedicated multistage hydraulic fracturing completions. The first well, located in the Central North Sea (referred to as ML-CNS), was stimulated using acid fracturing; while the second well, located in the Black Sea (referred to as ML-BKS), was stimulated using proppant fracturing. This paper presents the different drivers, challenges and lessons learned for each well while emphasizing the well construction and stimulation methodologies developed for the different reservoirs and field characteristics.
The field development drivers for drilling and completing these offshore hydraulic fractured multilateral wells, a first of their kind globally, was different for each case. The objective of the first project, initially considered uneconomic, was to engineer a technical solution for completion and production of two separate reservoirs with only one subsea well. The second project was seeking to optimize infill drilling from the last available slot on the offshore platform to maximize reservoir contact and production in the same reservoir. ML-CNS was a TAML Level 2 completion with a 14-stage, 5 ½" multistage completion run in each lateral and set-up for sequential acid fracturing. Operationally, the first lateral was drilled and stimulated, followed by the drilling and stimulation of the second lateral, using the drilling whipstock to navigate through the multilateral junction. ML-BKS was a TAML Level 3 completion that had a 6-stage, 4 ½" multistage completion installed in each lateral, which were proppant fractured following a sequence designed to minimize the jack-up rig time required. Both legs were drilled and completed prior to starting the stimulation, access to either lateral was achieved with the existing workover unit on the platform by manipulating a custom designed BHA.
The lessons learned from the first project executed in the North Sea were able to be transferred and applied to the second project in the Black Sea to allow for a more efficient and confident completion solution. Led by varying economical and regional constraints, the key factor for both wells centered on delivering operationally simple and reliable multilateral completion designs to economically meet the field development strategy in place.
To the knowledge of the authors and following subsequent literature research, both wells are a worldwide first for an offshore multilateral well completed with multistage acid fracturing and multistage proppant fracturing, and together they represent a new trend in cost-effective offshore field development through well stimulation. The successful case studies for both wells with the combined analysis of the benefits, challenges, and lessons learned will provide a guide and instill confidence with operators who find this approach beneficial with a view to applying it in other assets.
Tight gas well stimulation has a long and sporadic history in the North Sea and Europe and it is still far from being an easy development option in the current economic climate. Onshore, the pause in activity in Germany continues, but there appears to a light at the end of the tunnel, after recent new legislation has been passed. In Poland and Hungary, activity has fallen sharply, but there has been renewed interest in offshore tight gas in the Dutch and UK sectors (stranded gas). In addition to new field development, there is also potential development of tight gas horizons within existing fields. The economics of such developments are much more attractive than standalone tight gas projects, although there are often operational issues that need to be considered. This session will examine recent developments in tight gas stimulation, both onshore and offshore, and focus on case studies. The emphasis will be building on the recent experience to push the envelope of tight gas stimulation practices.
We describe a new (or under-reported) type of deformation feature that has some of the textural characteristics of both a fracture and a shear band. The examples described occur in experimentally-deformed source-rock materials, and in tight limestones, both of which are constituents of many shale reservoirs. The deformation features, which emerge at very low magnitudes of bulk strain, create new dilative zones within the rock, and thus enhance the flow characteristics. Direct observation of fluid flow, involving neutron-tomography experiments of these experimental samples, reveals flow behaviours that lead to the inference that the features have an unusual set of properties: both high capillary pressure and high permeability. Detailed textural observations generate insights that lead to hypothesized physical explanations for the surprising flow characteristics. Our present understanding is that these features can form in the low-strain (and low energy-cost) conditions that can be achieved in hydraulic stimulation operations. If such deformations do occur in the suitable rock types within shale sequences, their role in fluid flow may be significant but heretofore unrecognized.
Hydrocarbons in ‘shale’ reservoirs require stimulation to be effectively accessed, nominally by means of hydraulic fracturing (HF). There is growing appreciation that HF provokes a distributed response through the rock mass, involving rock breakage and movement over a large volume of the potential reservoir, including both displacements along natural fractures and new deformations. Thus, HF in unconventionals leads to the need to understand if deformation features, such as those studied herein, may be located within suitable rock types, and how the resulting textures and patterns of the features may impact fluid flow. Here, we describe the textural and property characteristics of experimentally-created ‘shear fractures’ in mudrocks and fine-grained carbonates, which are commonly components of the inter-layered sequences of some current shale plays.
The lab-induced deformations exhibit local dilational volume changes, and on that basis the local deformations would be expected to serve as flow conduits. In micro- and nano-scale investigations, however, the features are not seen as clean openings, as expected of ‘fractures’. Instead, they are filled with a newly-created ‘fault-rock’ equivalent material that has textures reminiscent of the gouge that occurs in shear-bands affecting siliciclastic rocks. Such shear bands in sandstones have historically been assumed to serve as flow barriers. However, in some of the examples here, the bands do operate as flow conduits – as revealed by direct flow observations using time-lapse neutron tomography experiments. The bands in the lab samples are inferred to be a result of local shear strain, plus or minus volumetric dilations or compactions. Numerical simulations of the experiments, which involve an enforced shear motion across an initially-intact layer, produce the same patterns of volumetric and shear strains inferred from the post-experiment textural examinations, and thus the simulations are judged as capturing the same operative phenomena, and the physical understanding that is derived from the simulations may be applied to the experimental outcomes. The emerging concept model is one in which localized shear features may develop in poor-quality rocks subjected to low values of bulk strain, creating previously-unanticipated flow pathways.
Chiotoroiu, Maria-Magdalena (OMV E&P) | Clemens, Torsten (OMV E&P) | Zechner, Markus (OMV E&P/Stanford University) | Hwang, Jongsoo (University of Texas) | Sharma, Mukul M. (University of Texas) | Thiele, Marco (Streamsim/Stanford University)
Waterflooding can lead to substantial incremental oil production. Implementation of water injection projects requires the project to fit into the risk (defined here as negative outcomes relative to defined project objectives) and uncertainty (defined here as inability to estimate a value precisely) a company is willing to take.
One of the key risks for water injection into a shallow reservoir is injection induced fractures extending into the caprock. If this risk is seen as "Intolerable" in an As Low As Reasonable Practicable (ALARP) analysis a decision may be made to not proceed with the project., In this study we evaluated caprock integrity by conducting simulations of long-term water injection that include the effects of formation damage caused by internal/external plugging, geomechanical stress changes and fracture propagation in the sand and bounding shale.
The risk of fracture growth into the caprock was assessed by conducting Monte-Carlo simulations considering a set of modelling parameters each associated with an uncertainty range. This allowed us to identify the range of operating parameters where the risk of fracture height growth was acceptable. Our simulations also allowed us to identify important factors that impact caprock integrity. To cover the uncertainty in geomechanical reservoir evaluation, the operating envelope is identified such that the risk of the caprock integrity is reduced. This requires introducing a limit for the Bottom Hole Pressure (BHP) including a safety margin.
The limit of the BHP is then used as a constraint in the uncertainty analysis of water injectivity. The uncertainty analysis should cover the various development options, the parametrisation of the model, sampling from the distribution of parameters and distance-based Generalized Sensitivity Analysis (dGSA) as well as probabilistic representation of the results.
The dGSA can be used to determine which parameter has a strong impact on the BHP and hence the project and should be measured if warranted by a Value of Information analysis.
The final development option to be chosen depends on a traditional NPV analysis.
The initial high cost of exploitation of the sustained, increasingly growing development of unconventional resources in Argentina has resulted in concentrating all efforts to increase well productivity while reducing construction and completion costs. The optimization of hydraulic fracture (HF) treatments is vitally important. It is the primary strategy used to achieve an optimal reservoir drainage area, consequently characterizing the fracture geometry, including the height, for the continuous improvement of HF treatment and planning.
Several types of technologies and methodologies are used to estimate fracture height during and after a hydraulic stimulation treatment. These technologies can provide information about the fracture geometry and extension in the near-wellbore (NWB) and far-field areas. The determination of a reliable correlation between those methodologies represents a challenge as a result of formation complexity, heterogeneity, and limitations of evaluation technologies. It is well-known that some areas in the Vaca Muerta formation contain layers that can act as fracture barriers and are responsible for fracture containment.
This paper presents a fast and simple methodology that uses conventional well logs [gamma ray (GR), sonic, and density] from pilot wells to identify potential fracture barriers. This approach establishes a means to evaluate the degree to which the rock will have the ability to control fracture height growth. This methodology was determined useful for planning perforation intervals or clusters placement, particularly in those formations with stress profile showing reduced stress contrast and, when complemented with geological information, this method also provides useful information for horizontal well trajectory. Case studies are provided to illustrate examples of the proposed fracture barrier index (FBI) being calibrated or compared to other fracture height assessment. Additionally, the benefits of adding this new approach to current methodologies and technologies to aid completion design optimization and decision making is discussed.
Rate transient analysis using log-log plots of rate-normalized pressure (RNP) and its derivative (RNP') versus material balance time have proven helpful in providing estimates of shale matrix permeability and SRV drainage volumes in multiple transverse fracture wells (MTFW's) (
We have constructed an analytical model of MTFW's that accurately predicts individual fracture flow performance for both constant and variable rate and constant bottom hole pressure inner boundary conditions. Using this model, we can accurately compute the pressure disturbance and rate change seen at the whole well and for individual fractures to quantify the degree of interference between fractures for any number of parallel, equally-spaced, and equally-sized fractures. This model has been validated by simulation using a commercial simulator. With both this analytical model and a series of numerical simulations, we investigated the fundamental mechanisms of flow in MTFW's and how the estimation of telf may be improved.
Previous authors have represented the progression of flow regimes in MTFW's as a linear flow period that transitions to a pseudo steady state (or apparently boundary-dominated) flow regime. We show that the same flow response is exhibited by a fully-infinite linear system, calling into question the nature of the "stimulated reservoir volume" (SRV) as a bounded reservoir system. In addition, we show telf can be detected and interpreted as the beginning of the onset of this fracture interference using the "limit of detectability" concept.
Numerical modeling of unconventional reservoirs, using commercial simulator, requires the construction of SRV that incorporates explicit geometries of primary and secondary fractures. Most of the time, during two phase flow, these models exhibit constant GOR. The method outlined in this paper eliminates the limitations of constant GOR being the only outcome and shows all other possible GOR responses that have a direct bearing on long term deliverability.
To overcome the drawback of numerical simulation, we make use of a dual porosity semi-analytical model. Using the concept of dimensional productivity index and its derivative, two-phase solutions are generated with the help of sandface pseudopressure. This model helps define all GOR variations of a ‘complete’ dual porosity system, wherein the fracture domain is assumed to contain part natural fracture and part induced hydraulic fracture. Apart from the impact of fluid variation, the other contributing factor in GOR variation is fracture density, which is addressed with the help of idealized fracture orientations such as the slab (planar 1D primary fracture only), the cylinder (non-planar 2D fracture with planar secondary fracture) and the sphere (non-planar 3D fracture with non-planar secondary fracture). These matrix shapes encompass all possible range of fracture densities (both natural and induced) that impact long-term performance.
The results of this method are shown for different fluids and for different idealized fracture orientations. It demonstrates that if the volume of fractures, in a given volume of SRV, is kept constant then the fluid type and fracture orientation are directly responsible for rate of pressure depletion that gives rise to these variations in GOR. It brings out the effect of different fracture orientations on the same fluid type and the effect of the same orientation on different fluid types. Additionally, with the help of published literature data, it is also demonstrated how GOR variation complements the conventional rate transient analysis for such unconventional reservoirs.
With the use of this method we can evaluate horizontal fractured well performance for different types of fluids and different fracture orientations in liquids-rich shale reservoirs. This GOR variation is important sensitivity parameter conveying the information about natural fractures present in the reservoir, information which is hard to get by from any other source, thus bringing out the significance of this method.
Acid fracture operations in carbonate formations are used to create highly conductive channels from the reservoir to the wellbore. Conductivity in calcite formations is expected to be highest near the wellbore, where most of the etching occurs. The near wellbore fracture etched-width profile can be estimated from the measured temperature distribution. Temperature data can be obtained from fiber optic distributed temperature sensing (DTS) installed behind casings to monitor fracturing operations.
Heat transfer is commonly coupled in acid fracture models to account for temperature's effects on acid reactivity with carbonate minerals. Temperature profiles are usually evaluated during simulations of fracture fluid injection, but seldom during fracture closure. Since most of the acid is spent during injection, many models have assumed that the remaining acid reacts proportionally along the fracture length. Because of this assumption, neither acid spending nor temperature is usually simulated during fracture closure.
In this study, a fully integrated temperature model was developed wherein both the acid reaction and heat transfer were simulated while the fracture was closing. At each time step, transient heat convection, conduction, and generation were calculated along the wellbore, reservoir, and fracture dimensions. Modeling temperature during this transient period provides a significant understanding of the fracture etched-width distribution. During shut-in, cold fracture fluids are heated, mainly because of heat flow from the formation to the fracture. The amount of fluid stored in the fracture determines how fast the fluid is heated. Wider fracture segments contain larger amounts of cold fracture fluids, resulting in it taking longer to reach the reservoir temperature. Because of this phenomenon, near a wellbore, the vertical fracture etched-width profile can be determined from the temperature distribution. Also, minerals' spatial distributions along the wellbore's lateral can be estimated in multistage acid fracturing. This is done by minimizing the difference between the observed and modeled temperatures.
This evaluation of etched width profiles at the fracture entrance provides an estimation of fracture-conductive channel locations. Moreover, it has significantly improved the understanding of mineralogy distribution in multi-layer formations. This information will be particularly useful when designing acid fracturing jobs in nearby wells or revisiting the same wellbore for further stimulation.
The technical and economic successes of deep geothermal development rely on reducing drilling and completion risks. In the closely related oil and gas activities, the risk taken by the investors is balanced by the high reward that successful projects achieve by immensely offsetting the losses of the failed wellbores. Geothermal projects experience similar risks, however, the potential reward is limited by the competition with other energy sources, in a heavily regulated market. The economic acceptability of geothermal power generation requires low risk drilling and completion technologies that would work under many different geological conditions.
When wells are drilled into a petro-thermal formation, sometimes referred to as hot dry rock (HDR), there is normally no clear circulation path between these wells and when this path exists, the transmissivity is so low that no economical project is possible. Enhanced geothermal systems (EGS), in these circumstances is closer to reservoir creation than to conventional reservoir stimulation. Therefore, developing technologies that achieve the designed EGS size and transmissivity is vital to deep geothermal development.
The EGS becomes a viable proposition, when enough rock surface can be contacted by the geothermal fluid, and when the flow path runs smoothly through a sufficient rock volume to minimize the energy depletion and have the project running over a long period, compatible with a positive net present value (NPV). To that end, the well design and its completion system have to be engineered to maximize the chances of properly creating the EGS. In this paper, lessons learnt from past geothermal experience are reviewed and analysed to propose a multi-stage system as a mean of improving geothermal wells completion reliability. Current oil and gas (namely "unconventional") completion technologies related to multi-stage stimulation have been reviewed and different options are discussed in the scope of a deep geothermal hot dry rock project. While previous works conclude that technologies developed for oil and gas are readily available and applicable to deep geothermal projects and EGS (Gradl, 2018), this study shows that shortcomings exist and that further developments are necessary to reliably and economically complete EGS projects. The necessary tests before running different parts is also discussed. Other options for reservoir creation are debated with their potential benefits and associated risks. The developments that could make them work in an EGS project are discussed.