Water Control in Oil Wells With Downhole Oil-Free Water Drainage and Disposal

Abstract

In bottom water drive oil reservoirs, the use of dual-completed wells with water drainage-dubbed: Downhole Water Sink (DWS) technology - has proved to control water coning, increase oil production rate and total recovery factor. Also, in field trials, the technology demonstrated sustainable production of oil-free drainage water. The ability to produce oil-free water makes DWS extremely attractive in offshore operations as no water processing is needed prior to overboard discharge. In land operations, oil-free water may be injected into the same well below the drainage completion or lifted to the surface and disposed into injection wells without processing. Field tests and our studies have also showed that sustainable drainage of oil-free water becomes somewhat difficult as the two completions (top and bottom) may receive co-mingled inflows of the two fluids. The phenomenon indicates the existence of transition zone (with mobile oil and water) much larger than that explained by the capillary pressure effect. This paper presents the results of a study - using numerical simulator and a pie-shaped physical model- aimed at understanding the change of transition zone around producing wells with and without DWS. The results show that, in conventional wells with water coning, the transition zone is small and constant away from the well but enlarges towards the wellbore. This transition zone enlargement effect occurs in conventional wells due to diffusion resulting from pressure distribution around the well. For DWS wells, however, the effect is not only more pronounced but it also alters the IPW plots - a basic tool for design. There is a commingled inflow envelope in addition to the envelope of segregated inflow. Based on the above understanding, the paper shows how to design operational limits for DWS wells to maximize oil production from the top completion and maintain oil-free water drainage from the bottom completion. Introduction Production of oil and gas from bottom water drive reservoirs involves aquifer encroachment resulting in production of water. The extent to which produced water is a problem has been estimated to cost the petroleum industry about $45 billion a year1. These costs include the expense to lift, dispose or re-inject produced waters, as well as the capital investment in surface facility construction and to address other environmental concerns1. In fact, Kimbrell2 asserted that, "Produced water is a fact of life in Louisiana. The largest volume of waste associated with oil and gas production operations in Louisiana, as well as nationally, is produced water". In 1993, over 1.2 billion barrels of produced water was generated compared to less than 200 million barrels of oil and condensate and a little over 200 million BOE (barrel oil equivalent) of gas produced in the same period. From 1990 to 1993, the statewide water-hydrocarbon ratio (WHR) averaged approximately 3.2"2. Various governments around the world have discharge limits for residual oil in produced water that range from 23–40 mg/l. Some have also imposed zero discharge limits, which in practice, demands re-injection of produced water. "Average oil content of produced water in Norway in 1998 was 23 mg/l, giving an oil discharge of about 2100 tonnes from about 100 million tonnes of total annual produced water. Aromatic compounds like Benzene and solubles such as Phenols made up additional 1200 tonnes. Polyaromatic Hydrocarbons (PAH) and Alkylphenols cause the most worry, aand mounted up to 49 tonnes in 1998"3. In the United States, the National Pollutant Discharge Elimination System (NPDES) regulates the requirements regarding maximum hydrocarbon contamination for offshore produced water disposal in the United States. This produced water disposal requirements depend solely on the level of hydrocarbon contaminants as in the OCS (Outer Continental Shelf) of the Gulf of Mexico and in the Niger Delta. Table 1 gives a brief overview of the requirements for maximum concentration of hydrocarbons.

A Novel Automatic Inflow-Regulating Valve for Water Control in Horizontal Wells

“…Because of reservoir heterogeneity and the “heel-toe” effect, the problems of water coning, short water breakthrough time, and imbalanced production profiles have become the major difficulties that restrict the development of horizontal wells (Figure ). The six main methods of solving these problems are the maximum water-free production rate, annular zone isolation using chemicals, , variable density perforation, , dual horizontal well completion, − stinger completion, , and inflow control device (ICD)/automatic inflow control device (AICD) completion technology. − Figure shows a schematic of ICD/AICD technology, which has been the most popular method in Chinese oil fields over recent years.…”

Loss and restoration of water coning control – field case history and prediction

“…Similar mechanism is employed in the DWS technology for local control of water coning [34]. Generally, a DWS well has two completions: the top completion is installed in the oil pay zone to produce oil, and the bottom completion is placed in the aquifer to keep the oil water contact (OWC) beneath the top completion [35][36][37][38][39][40]. To date, various versions of DWS technique have been developed by the Downhole Water Sink Technology Initiative (DWSTI) research program at the Louisiana State University (LSU) -including Downhole Water Loop (DWL), and Bilateral Water Sink (BWS) etc.…”