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A challenge in oil-reservoir studies is evaluating the ability of geomechanical, statistical, and geophysical methods to predict discrete geological features. This problem arises frequently with fracture corridors, which are discrete, tabular subvertical fracture clusters. Fracture corridors can be inferred from well data such as horizontal-borehole-image logs. Unfortunately, well data, and especially borehole image logs, are sparse, and predictive methods are needed to fill in the gap between wells. One way to evaluate such methods is to compare predicted and inferred fracture corridors statistically, using chi-squared and contingency tables.
In this article, we propose a modified contingency table to validate fracture-corridor-prediction techniques. We introduce two important modifications to capture special aspects of fracture corridors. The first modification is the incorporation of exclusion zones where no fracture corridors can exist, and the second modification is taking into consideration the fuzzy nature of fracture-corridor indicators from wells such as circulation losses. An indicator is fuzzy when it has more than one possible interpretation. The reliability of an indicator is the probability that it correctly suggests a fracture corridor. The indicators with reliability of unity are hard indicators, and “soft” and “fuzzy” indicators are those with reliability that is less than unity.
A structural grid is overlaid on the reservoir top in an oil field. Each cell of the grid is examined for the presence and reliability of inferred fracture corridors and exclusion zones and the confidence level of predicted fracture corridors. The results are summarized in a contingency table and are used to calculate chi-squared and conditional probability of having an actual fracture corridor given a predicted fracture corridor.
Three actual case studies are included to demonstrate how single or joint predictive methods can be statistically evaluated and how conditional probabilities are calculated using the modified contingency tables. The first example tests seismic faults as indicators of fracture corridors. The other examples test fracture corridors predicted by a simple geomechanical method.
Hu, Zhenhua (PetroChina Liaohe Oilfield Company) | Zhang, Shenqin (PetroChina Qinghai Oilfield Company) | Wu, Fangfang (Schlumberger) | Liu, Xunqi (Schlumberger) | Wu, Jinlong (Schlumberger) | Li, Shenzhuan (Schlumberger) | Wang, Yuxi (Schlumberger) | Zhao, Xianran (Schlumberger) | Zhao, Haipeng (Schlumberger)
The igneous reservoir of Shahejie formation in eastern sag of Liaohe depression is characterized by complex geological environment, variable lithology and high heterogeneity. Reservoir evaluation is difficult only based on conventional logs due to complex lithology and pore structures. Effective igneous reservoirs were identified and reservoir controlling factors were analyzed based on effective porosity calculation, pore structure analysis, lithology identification, lithofacies analysis, fracture evaluation and heterogeneity analysis by combing nuclear magnetic resonance data, micro-resistivity image data, conventional logs as well as mud logging data.
Based on our study, the igneous reservoirs in the study area are more related with effective porosity and pore connectivity, and less related with fractures. Good reservoirs are mainly distributed on the top part of explosive facies and effusive facies, where lithologies are mainly Trachyte, volcanic breccia and breccia-bearing tuff. The weathering leaching process is quite important for igneous reservoirs, but the reservoir qulity would not be good if the weathering process is too strong as it will lead to low effective porosity.
The accuracy of igneous reservoir evaluation gets improved a lot by this integrated approach and the conclusion from this study will help to optimize igneous reservoire exploration plan.
A pre-exploration well was drilled in the Xihu Sag of East China Sea basin, and commercial oil and gas flow had been achieved. But the oil and gas bearing trap had a big depth with low closure height and small area. The resolution of seismic data acquired by towed streamer is low, so it's difficult to obtain seismic velocity precisely. There were great risk and uncertainty in description of the trap and distribution of gas-bearing sandstone, reservoir prediction of sweet spot, direct hydrocarbon indication, and reserves assessment.
In consideration of the drilling platform on the trap, seismic acquisition technique of walkaway VSP and walk around VSP were introduced, meanwhile some innovative methods in source, receivers and geometry were applied. Twenty three-component hydrophones were composed as signal receivers which had a sample interval of ten meters in the well, two straight shot lines and two loop shot lines were designed around the drilling platform. Besides, volume and depth of air gun array were optimized, and the sailing route of seismic source vessel was planned properly in order to improve the efficiency of collecting work.
The collecting work of walkaway VSP and walk around VSP was accomplished efficiently, and more than seventy kilometers VSP seismic data was achieved. Afterwards, the new data was processed finely in company with zero offset VSP data, so high resolution VSP profiles and accurate seismic velocity were obtained. Reprocess to original seismic data acquired by towed streamer was implemented on the basis of walkaway VSP and walk around VSP data. The quality of normal seismic data was improved through reprocess constrained by walkaway VSP data, and S/N and resolution were much higher than old data. So it would be credible to research the distribution of gas-bearing sandstone and direct hydrocarbon indication using the reprocessed seismic data.
It was the first time to use joint acquisition technique of walkaway VSP and walk around VSP in offshore China which was an important breakthrough. High resolution VSP seismic profiles and precise seismic velocity could be acquired, and the data was important basis for refined evaluation of pre-exploration targets. It's very necessary to popularize and utilize these new techniques further.
Chen, Zhiming (State Key Laboraory of Petroleum Resources and Prospecting, China University of Petroleum at Beijing) | Xie, Jianyong (Xinjiang Oilfield Corporation, PetroChina) | Liao, Xinwei (State Key Laboraory of Petroleum Resources and Prospecting, China University of Petroleum at Beijing) | Li, Xiaofeng (Changqing Oilfield Corporation, PetroChina) | Zhang, Jiali (State Key Laboraory of Petroleum Resources and Prospecting, China University of Petroleum at Beijing) | Li, Rongtao (State Key Laboraory of Petroleum Resources and Prospecting, China University of Petroleum at Beijing) | Li, Lang (Liaohe Oilfield Corporation, PetroChina)
In this study, we develop a new model for the complex fracture geometries with fracture hits using an efficient semi-analytical model. The semi-analytical model has the capability to simulate shale well performance by considering complex non-planar hydraulic fractures and fracture hits. By combining nodal analysis and Laplace transforms, the pressure transient solution of the diffusivity equation is obtained. The semi-analytical model is verified against the numerical models. Then, we apply the model to analyze pressure testing data of a parent well and a child well. Results show that the flow regimes of complex fracture geometries with fracture hits include wellbore storage, skin effect, fracture bilinear flow, "fluid feed", pseudo-boundary dominated flow, unconnected fracture (UF) impact, and pseudo radial flow. During the flow regime of UF impact, the pressure derivative curves exist a second "V-shape", as the UF improve the matrix permeability and the pressure depletion will be reduce once the flow reaches the UF. Those flow regimes provide good guidelines for identification of complex fracture networks with fracture hits.
In this work, we show a case study of using fire flooding process in thin interbeded heavy oil reservoirs to tackle the problems of low oil production rate, low gas oil ratio, and low oil recovery factor in later stage of cyclic steam stimulation (CSS) operations. The fire flooding process was adopted for the Block D reservoir in Liaohe Oilfield, China.
The depth of Block D heavy oil reservoir is 800-1200m. The reservoir includes 30-40 thin layers in vertical direction, with average thickness of 2.2m per layer. Through laboratory experiments, numerical simulation, and analysis of surveillance data from the initial small scale field pilot, we improve the understanding of fire flooding recovery mechanisms in thin interbeded reservoirs. Then, technical limits such as reservoir thickness, number of layers and permeability contrast are established for successfully conducting fire flooding with high temperature oxidation. Reaction types identification methods based on corresponding field surveillance data are also proposed. Using such guideline, we have deployed fire flooding expansions in Block D reservoir with more well patterns.
It has been proven that fire flooding process with the designed well patterns can improve recovery and sweep, and make the wells more productive with enhanced inflow from multiple directions. The process technical limits successfully guides us in properly expanding the project. Reaction type identification method further helps to perform continuous dynamic surveillance of the high/low temperature oxidation burning state of the combustion front in the field. The initial pilot test includes 7 well patterns, which have been operational since 2005. Up to now, we have deployed a total of 105 fire flooding well patterns in Block D reservoir. Within the fire flooding area, the single-well oil production rate has doubled on average, with the reservoir pressure also doubled. For the multi-layered block D reservoir, the ultimate oil recovery factor for fire flooding can reach up to 55%, an increase of 28% from the expected recovery of CSS.
In conclusion, we have shown in this work that fire flooding process can be applied to deep thin to medium thickness interbeded heavy oil reservoirs. As a follow-up process in later stage of cyclic steam stimulation, it can significantly increase oil recovery and process performance.