Asia's first rigless subsea stimulation was executed in 2018, with intervention performed upon three target wells offshore Sabah Malaysia, at a water depth of approximately 1400 m (4,593 ft). Significant changes in reservoir performance prompted an acid stimulation and scale squeeze treatment, designed to remedy fines migration and scaling issues within the well and production system. Treatment fluids were delivered subsea by an open-water hydraulic access system, using a hybrid coiled-tubing downline. Access to the subsea trees was permitted via a patented choke access technology, allowing for a flexible, opex-efficient, and low-risk intervention. The intervention system was installed upon a multi-service vessel, with the downline deployed via the vessel moonpool. A second support vessel was used as required to provide additional fluid capacity without disturbing primary intervention operations. This enhanced the flexibility of the operation, permitting changes in the treatment plan to be accommodated for without impact to critical path stimulation activities.
The full intervention was delivered as an integrated service, with all elements supplied by a single provider, via one contract. An established network of in-house equipment, expertise, test laboratories, and operational bases supported the planning and execution of the project. This was complemented by select external providers for vessels, remotely operated vehicle services, and other specialist contractors.
The challenges faced during this new market entry included completion of a comprehensive treatment fluid test program, importation and logistics of equipment from around the globe, and managing operational risks, all within a condensed timeline to satisfy a brief intervention window. By leveraging the diverse global network of the service provider, the technology and people required for the project were accessed and brought together to achieve a collaborative solution. This was enhanced by the inclusion of performance based elements within the contract. The provision of a highly efficient and flexible well access technology also supported rapid mobilization and operational risk reduction.
Post-stimulation well testing confirmed an average increase in oil productivity of 86%, with a corresponding productivity index factor (PIF) gain of 3.4. These results, combined with the efficient execution of the campaign, confirm the appropriateness of open-water hydraulic access using coiled-tubing for performing cost-effective stimulations on complex subsea wells.
Successful entry to the region was highly dependent upon the integrated nature of the service. Access to the service providers global network permitted a high degree of influence upon the ultimate performance of the stimulation. Examples include the PIF results achieved and the responsive actions taken to remedy offshore challenges such as reservoir lock-up on well #3.
We have seen that two data sets can have the same univariate statistics, yet have very different spatial properties (Figure 1). The complex attributes we deal with in the petroleum industry can be described by random functions that are combinations of regionalized and random variables. Regionalized variable theory is based on the statistics of the RV,    which differs from ordinary scalar random variables in its spatial continuity, yet still possesses the usual distribution statistics, such as mean and variance. The RV also differs in that it has a defined location.
It is often necessary to investigate the forecast uncertainty for a portfolio of fields or reservoirs to evaluate, for example, the risks and opportunities of an exploration portfolio, of a new business strategy, for an "urban planning" study or to evaluate uncertainty in the regional portfolio. It is important to understand whether the portfolio uncertainties are dependent or independent. Cases 1, 2 and 3 are often dependent with complex interactions of the parameters and strong inter-dependencies (both positive and negative correlations) and with common system constraints. In this case, a comprehensive Monte Carlo analysis of the system is recommended that includes all the complex system interactions. Aggregation tools are available in the industry to do this complex probabilistic aggregation after individual field forecasts have been generated; however, most IPSM tools have the ability to evaluate the system uncertainty for all assets concurrently and the latter approach would be preferred, but is sometimes considered too time-consuming.
Valuations of oil and gas properties are needed for many of the same reasons appraisals are needed for homes, cars, jewelry, or any other assets. Lenders require some type of valuation when assets are used as collateral for a loan. Taxes are often assessed on the basis of property value. Property values have a bearing on rates for insurance policies and settlements after loss, damage, or foreclosure. Economic evaluations are seldom made simply for curiosity. In most cases, they are needed for some business reason. The primary method of evaluation discussed in this chapter is the deterministic cash flow method. Projected schedules of quantities of reserves from a deterministic reserves study projected in selected future time frames (usually calendar years) are displayed in the results, but computations are often made monthly. Schedules of future prices and costs are projected for the same time frames. Volumes are multiplied by prices, and costs are deducted to estimate future net revenue (FNR). Present worth calculations to factor in the time value of money are applied to the projected cash flow stream, and the results are reported. A table presenting a suite of present worth values over a range of discount rates is usually included. Individual projections are usually made by well or ownership entity. Each projection is classed in a reserves category (i.e., proved, probable, or possible) and more specific subclasses are common. The results are summarized at the reserves category level. This valuation procedure is based on the methods used to estimate reserves in the chapter on the estimation of oil and gas reserves in this section of the Handbook. Within the limits of the reserves category definitions, the parameters used in the estimation of reserves are generally average values. In the case of proved reserves, the results of the calculation are perceived to be best estimates.
A passive tracer that labels gas or water in a well-to-well tracer test must fulfill the following criteria. It must have a very low detection limit, must be stable under reservoir conditions, must follow the phase that is being tagged and have a minimal partitioning into other phases, must have no adsorption to rock material, and must have minimal environmental consequences. The tracers discussed in the following sections have properties that make them suitable for application in well-to-well test in which dilution volumes are large. For small fields in which the requirement with respect to dilution is less important, other tracers can be applied. Figure 1.1 – Production curve of S14CN compared with the production curve of HTO in a dynamic flooding laboratory test (carbonate rock) (after Bjørnstad and Maggio). There are no possibilities for thermal degradation, and it follows the water closely. The 36Cl- is a long-lived nuclide (3 105 years), and the detection method is atomic mass spectroscopy rather than radiation measurements. The disadvantage is that the analysis demands very sophisticated equipment and is relatively time consuming. For mono-valent anions, the retention factors (see Eq. 6.2) are in the range of 0 to -0.03, which means that such tracers pass faster through the reservoir rock than the water itself (represented by HTO). A compound such as 35SO42- may be applied in some very specific cases but should be avoided normally because of absorption. Some anionic tracers may show complex behavior. Radioactive iodine (125I- and 131I-) breaks through before water but has a substantially longer tail than HTO. Both a reversible sorption and ion exclusion seem to play a role here. Cationic tracers are, in general, not applicable; however, experiments have qualified 22Na as an applicable water tracer in highly saline (total dissolved solids concentration seawater salinity) waters. In such waters, the nonradioactive sodium will operate as a molecular carrier for the tracer molecule. Retention factor has been measured in the range of 0.07 (see Eq. 6.2) at reservoir conditions in carbonate rock (chalk). Wood reported the use of 134Cs, 137Cs, 57Co, and 60Co cations as tracers.
Estimating resource and reserves crosses the disciplines between geoscientists and petroleum engineers. While the geoscientist may well have primary responsibility, the engineer must carry the resource and reserve models forward for planning and economics. Volumetric estimates of reserves are among the most common examples of Monte Carlo simulation. Consider the following typical volumetric formula to calculate the gas in place, G, in standard cubic feet. In this formula, there is one component that identifies the prospect, A, while the other factors essentially modify this component.
Nuclear magnetic resonance (NMR) has been, and continues to be, widely used in chemistry, physics, and biomedicine and, more recently, in clinical diagnosis for imaging the internal structure of the human body. The same physical principles involved in clinical imaging also apply to imaging any fluid-saturated porous media, including reservoir rocks. The petroleum industry quickly adapted this technology to petrophysical laboratory research and subsequently developed downhole logging tools for in-situ reservoir evaluation. NMR logging, a subcategory of electromagnetic logging, measures the induced magnet moment of hydrogen nuclei (protons) contained within the fluid-filled pore space of porous media (reservoir rocks). Unlike conventional logging measurements (e.g., acoustic, density, neutron, and resistivity), which respond to both the rock matrix and fluid properties and are strongly dependent on mineralogy, NMR-logging measurements respond to the presence of hydrogen protons.
This glossary was created through discussions among the steering committee for the SPE Global Integrated Workshop Series (GIWS) on Production Forecasting. Some definitions were not contested at all, others generated fierce discussions. The contract quantity is the contractually agreed volumes and limits: predefined (annual) volume of natural gas on contract level. A factor applied to forecasts to take into account the fact that a Production System will not always operate at 100% of its capacity. Available But Not Required, that part of the IPSC that is available for production but not produced because of low off-take demand.
Both the computation of classical statistical measures (e.g., mean, mode, median, variance, standard deviation, and skewness), and graphic data representation (e.g., histograms and scatter plots) commonly are used to understand the nature of data sets in a scientific investigation--including a reservoir study. A distinguishing characteristic of earth-science data sets (e.g., for petroleum reservoirs), though, is that they contain spatial information, which classical statistical descriptive methods cannot adequately describe. Spatial aspects of the data sets, such as the degree of continuity--or conversely, heterogeneity--and directionality are very important in developing a reservoir model. Analysis of spatially rich data is within the domain of geostatistics (spatial statistics), but a foundation in classical statistics and probability is prerequisite to understanding geostatistical concepts. Sampling also has proved invaluable in thousands of studies, but it, too, can lead to statistical insufficiencies and biases.