Abstract
Shunbei Oilfield is characterized by substantial heterogeneity and a complex oil–water relationship. The water-oil interface is dynamically changing, and it is a crucial parameter for reserve calculation and evaluation. The main purpose is to analyze the effect of fluid flow in multi-scale media on the water-oil interface. It is well known that the fracture-cavity reservoirs have well-developed fractures and karst caves, and their distribution is complex in Shunbei Oilfield. This paper presents a way to simplify the fracture-cavity system first, then uses a unit of oil wells as a system to study the water-oil interface, which avoids impact on the water-oil interface due to oil production. A detailed step by step procedure for solving the semi-analytical solution of water-oil interface in a fracture-cavity reservoir by using an explicit algorithm and a successive steady-state method is presented. The solution can be used to investigate water-oil interface behavior. In this paper, we validated this method with the actual data for a relatively similar actual reservoir. Sensitivity analyses about the effects of the main parameters including production rates, cave volume and initial oil–water volume ratio on interfacial migration velocity are also presented in detail. The water breaking time of oil wells is fully investigated. The water-oil interface movement chart under different development conditions is established to predict the water-oil interface in the late stage of oil well production and extend the waterless developing period. Being based on this chart, a water breakthrough warning can be realized, and oil recovery can be improved. The findings of the research have led to the conclusion that the rising speed of water-oil interface is proportional to the production rate, on the contrary, it is inversely proportional to cave volume and initial oil–water volume ratio. As well production goes on, the water-oil interface rises at different rates. After the well is put into production for one year, the water-oil interface rises by 16.38%, 12.56% and 4.24% according to the condition that production rate is 10%, the initial oil–water volume ratio is 0.7, and the cave volume is 100 × 104 m3. This method is not only suitable for any period and any well type in the development of Shunbei Oilfield; it also has the function of calculating the real-time water-oil interface of a single well and multi-wells. This new method has the characteristics of easy calculation and high accuracy. The method in this paper can be further developed as it has great applicability in fracture-cavity reservoirs.
Highlights
The contact surface between oil and water is called water-oil interface (WOC). It is of great practical significance for reservoir development to accurately understand the depth of WOC in different oil wells and formulate reasonable development countermeasures [2]
For sandstone reservoirs, but few research results have been reported at this point concerning carbonate fracture-cavity oil reservoirs, especially the Shunbei Oilfield
The logging method has certain limitations when applied to fracture-cavity reservoirs, namely when the oil well has leaked, the logging operation cannot be performed at all
Summary
Due to the gravity differentiation and adjustment in a reservoir, oil occupies the high part of the reservoir [1], and water is located at the bottom or edge of the reservoir. The contact surface between oil and water is called water-oil interface (WOC). It is of great practical significance for reservoir development to accurately understand the depth of WOC in different oil wells and formulate reasonable development countermeasures [2]. For the Shunbei Oilfield, the early oil and gas charging mainly migrated upward along the fault and adjusted laterally [3]. Due to the unique storage space of fracturedcavity reservoirs, oil and gas are generally concentrated in caves, fractures or dissolved
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