Cosan Ayan

A New Pressure/Rate-Deconvolution Algorithm To Analyze Wireline Formation-Tester and Well-Test Data

Reconstructing constant-rate drawdown-pressure response and its logarithmic time (or Bourdet) derivative by deconvolution from multirate pressure-transient data is very important for wellbore-/ reservoir-system identification and interpretation. In recent years, the use of pressure/rate deconvolution has increased considerably because of significant improvement of the algorithms. In this paper, we present a new deconvolution algorithm based on a weighted Euclidean norm in the Tikhonov (1963) regularized objective function so that one can assign weights to individual pressureand rate-measurement points, and, thus, define different error estimates for different sections of the data. Incorporating such features into the deconvolution algorithm is very useful to mitigate the effects of unreliable pressure and rate measurements and the sections of the data not obviously consistent with the wellbore/reservoir model. We present two applications of the new algorithm using real field pressure/rate transient data sets. In addition to conventional drillstem-test (DST) well-test data, we apply the algorithm to wireline formation-tester (WFT) pressure transients, which are usually also referred to as interval pressure-transient tests (IPTTs). The results show that the new deconvolution algorithm presented in this paper is useful in interpreting pressure/rate transient data from both formation and well tests.

Mapping and Modeling Large Viscosity and Asphaltene Variations in a Reservoir Undergoing Active Biodegradation

Viscosity is one of the key reservoir fluid properties. It plays a central role in well productivity and displacement efficiency and has a significant impact on completion strategies. Accurately assessing areal and vertical variations of viscosity will lead to more realistic reservoir simulation and optimal field development planning. Downhole fluid analysis (DFA) has successfully been used to measure the properties of reservoir fluids downhole in real time. DFA has excellent accuracy in measuring fluid gradients which in turn enable accurate thermodynamic modeling. Integration of DFA measurements with the thermodynamic modeling has increasingly been employed for evaluating important reservoir properties such as connectivity, fluid compositional and property gradients. The thermodynamic model is the only one that has been shown to treat gradients of heavy ends in all types of crude oils and at equilibrium and disequilibrium conditions. In addition, fluid viscosity depends on concentration of heavy ends that are associated with optical density measured by DFA. Therefore, mapping viscosity and optical density (heavy end content) is a new important application of DFA technology for use as assessment of reservoir architectures and a mutual consistency check of DFA measurements. In this case study, a very large monotonic variation of heavy end content and viscosity is measured. Several different stacked sands exhibit the same profiles. The crude oil at the top of the column exhibits an equilibrium distribution of heavy ends, SARA and viscosity, while the oil at the base of the oil column exhibits a gradient that is far larger than expected for equilibrium. The fluid properties including SARA contents, viscosity and optical density vary sharply with depth towards the base of the column. The origin of this variation is shown to be due to biodegradation. GC-chromatographs of the crude oils towards the top of the column appear to be rather unaltered, while the crude oils at the base of the column are missing all n-alkanes. A new model is developed that accounts for these observations that assumes biodegradation at the oil-water contact (OWC) coupled with diffusion of alkanes to the OWC. Diffusion is a slow process in a geologic time sense accounting for the lack of impact of biodegradation at the top of the column. An overall understanding of charging timing into this reservoir and expected rates of biodegradation are consistent with this model. The overall objective or providing a 1st-principles viscosity map in these stacked sand reservoirs is achieved by this modeling. Linking DFA with thermodynamic modeling along with precepts from petroleum systems modeling provides a compelling understanding of the reservoir.

Flow Modeling and Comparative Analysis for a New Generation of Wireline Formation Tester Modules

Wireline formation testing (WFT) is an integral part of reservoir evaluation strategy in both exploration and production settings worldwide. Application examples include fluid gradient determination, downhole sampling, fluid scanning in transition zones, as well as interval pressure transient tests (IPTTs). Until recently, however, formation testing was still challenging and prone to failure when testing in low-mobility, unconsolidated, or heavy-oil-bearing formations, especially with single-probe type tools. A new-generation WFT module with a 3D radial probe expands the operating envelope. By using multiple fluid drains spaced circumferentially around the tool, the new module can sample in tighter formations and sustain higher pressure differentials while providing mechanical support to the borehole wall. We performed a detailed flow modeling-based analysis of the contamination cleanup behavior during fluid sampling with the new module. Using both miscible (sampling oil in oil-based mud) and immiscible (sampling oil in water-based mud) contamination models we studied the cleanup behavior over a wide range of formation properties and operating conditions. Comparison of the cleanup performance of the new module with the performance of conventional single-probe tools demonstrates that the new module is 10 to 20 times faster than the single-probe tools when sampling in tight formations. Finally, we also compared the new module against the sampling performance of dual packers and a focused probe. This work is directly relevant to the planning and fundamental understanding of wireline fluid sampling. The key contributions are miscible and immiscible contamination cleanup models that include the effect of tool storage, a comprehensive analysis of contamination cleanup behavior for the new-generation WFT module with comparisons against conventional single-probe, focused probe, and dual-packer tools, and a characterization of fluid sampling conditions versus the preferred type of sampling tool. Introduction A logical start for any wireline formation testing (WFT) operation is a tool string design that considers the formation evaluation objectives and expected formation and fluid properties. With the current availability of an arsenal of probes having different shapes, focused probes of circular or elongated design, and dual packers, this planning stage has now become a more complex process. The recently introduced 3D radial probe (Al Otaibi et al. 2012; Flores de Dios et al. 2012) adds another choice for the engineers in planning WFT surveys. Successful WFT operations demand that toolstrings be designed to meet specific formation and fluid challenges (Weinheber et al. 2008). The selected downhole pump and probe or dual-packer combination must be able to induce and maintain flow from the formation without causing excessive drawdown to stay above expected phase-separation envelope. It must achieve and keep a seal with the borehole face and must not plug during the cleanup operation. While achieving these performance goals, it must work effectively with the downhole pump to deliver high rates that reduce cleanup time. For transient testing, the tool system must have a small storage volume, flow should be smooth and stable, and buildup transients should be free of bore-hole or tool-induced noise. A major challenge for downhole sampling and downhole fluid analysis (DFA) is mud filtrate contamination. Acquired samples must be of sufficiently low contamination for reliable laboratory analysis, as well as better DFA. High miscible contamination (oil/gas and oil-based mud filtrate, water and water-based mud filtrate) or emulsions formed in immiscible fluids (oil and water-based mud filtrate, or water and oil-based mud filtrate) can make acquired samples unusable and reliable DFA may not be possible. Knowledge of tool interaction with the sandface, pump selection, flowing time and rate, and

Extending Formation Tester Performance to a Higher Temperature Limit

Exploration and development wells are increasingly drilled to deeper depths and lower porosities, in hotter formations. These conditions increase the challenges for Formation Tester’s (FT’s) to acquire accurate formation pressures in a timely and cost efficient manner. One of the most important constraints on current FT acquisition is the downhole formation temperature.

Pressure-Pressure Deconvolution Analysis of Multi-Well Interference and Interval Pressure Transient Tests

Following the introduction of pressure-rate (p-r) deconvolution techniques in 1960s, they have been investigated continually during the last two decades. Now most p-r deconvolution techniques are stable and have been used increasingly for analysis of pressure transient tests. A successful p-r deconvolution can transform a multirate pressure transient response to an equivalent constant-rate drawdown response for the entire test duration. It can help identify/confirm interpreted reservoir models and eliminates multirate superposition effects. However, the flow rate is usually not directly and continuously measured in conventional pressure-transient well tests, although it is measured reasonably accurately during interval pressure transient tests (IPTT) conducted by wireline formation testers (WFTs). Nevertheless, large uncertainties or errors associated with inaccurate rate data usually hinder the successful use of p-r deconvolution methods.