RIASSUNTO
Abstract
The paper describes a successful implementation of coiled-tubing gas lift (CTGL) in the diatomite formation of the Belridge field in Kern County, Calif.
The paper shows how the use of CTGL reclaimed 2000 BOPD that would have been otherwise lost due to irreparable casing damage. As soon as casing damage is detected, the future economic life expectancy of a well decreases to approximately 2 years. Wells with confirmed casing failure must generally be abandoned and redrilled, increasing costs significantly.
The paper will detail candidate well selection, equipment design and specifications. During the program, numerous design challenges had to be overcome. Special wellhead configurations were designed in order to ensure a safe and environmentally friendly operation. Coiled tubing (CT) selection was critical - both to optimize lift volume with minimized injection gas and to insure proper functionality and longevity in a high CO2 corrosive environment. The implementation of the project as well as associated risks and final production results will be mentioned as well.
Introduction
The Belridge field produces light crude from the diatomite formation from approximately 3600 wells. These wells are almost exclusively artificially lifted. Production is from a tight reservoir with a high porosity and low reservoir pressure due to the maturity of the field. Field development accelerated with the start of fracturing operations in the late 70's.
The field is developed on two anticlines which are trending northwest and are divided by the Middle Belridge fault. The length of the field is approximately 20 miles and the width is about 4 miles.
The diatomite target interval produces 10-340 API gravity sweet crude oil from depths of about 800 to 3,000 ft.[1] The gravity of the oil depends heavily on the geographic well location in the anticline, with the majority of the wells producing in the 21-340 API range.
The diatomite reservoir is the result of cyclic variations in depositional environments that yielded a series of stacked biogenic silica rich reservoirs.[2] Diatomite is characterized by two different layers, the opal A (amorphous, colloidal silica), with porosity ranges from 50-70%, and the opal-CT (cristobalite-trydomite), with porosity <45%. The amorphous structure of diatoms is changed due to depth increasing pressure and temperature. However, permeability is very low and ranges from 0.01-1 md. Formation thickness is 300 to 2,000 ft depending on the geographical location of the well in the anticline.
All diatomite producers are stimulated with hydraulically induced fractures to increase productivity. On average, a diatomite well has three fracture stages, each about 400 ft in height. The primary lift method is rod pumping with a conventional positive displacement pump. The diatomite gas-oil ratio (GOR) is generally low. Due to the maturity of the field, water injection is used primarily for pressure maintenance but also for drive support. Gross well production in the field averages between 60 and 300 B/D - with a high water cut of nearly 80%. Average well production is 20 BOPD after production stabilizes within a couple of years but averages 90 BOPD after initial completion.
Casing failures occur in wells producing from the diatomite reservoir due to several causes. However, as soon as casing damage is initially detected, the life expectancy of a well decreases to about 2 years depending on the geographic location. Wells with casing related problems must generally be abandoned and replaced, increasing operating costs significantly. Casing damage can be detected by observing several signs during workover operations. An early warning sign of pending casing failures is an increase in tubing and sucker rod related problems occurring at the same approximate depth. Also, a decrease in mean-time-between-failures due to tubing and rod failures indicates casing damage and is easily tracked and evaluated using graphical statistical evaluation methods (Fig. 1).