Pilot Steam Soak Operations in Deep Wells in China
Authors: Liu Wen-zhang 
Published: December 2007

Most the heavy-oil deposits in eastern China are buried fairly deep. Laboratory research and field pilot tests on four relatively deep oil reservoirs have been carried out since 1982. Gaosheng reservoir is 1500 to 1700 m [4,921 to 5,576 ft] deep and the other three reservoirs are 1000 to 1300 m [3,281 to 4,265 ft] deep. Viscosity values of oils at formation temperature are 1000 to 10 000 mPa [1,000 to 10,000 cp]. By the end of May 1985, 123 steamsoak operations had been carried out on 101 wells, with a cumulative oil production of 330 × 103 Mg [330,000 tonnes] and the corresponding amount of steam injected was 220 × 103 Mg [220,000 tonnes]. These operations were carried out mainly in three oil fields in the Liaohe area. These three fields with reservoirs at 1000 to 1300 m [3,281 to 4,265 ft] were almost nonproductive before, but generally a peak production of 100 to 200 Mg/d [100 to 200 tonnes/D] per well was obtained after steamsoak. Although low-quality steam was injected in the wells in the Gaosheng oil fields, good stimulation responses were still obtained.

Physical modeling and mathematical simulation on the wellbore heat transfer show that steam injection is feasible in the four oil fields by use of heat insulation techniques in the wellbores, and also is feasible for long-term steamflooding. This paper presents the results of optimization of the steam injection parameters by physical modeling and mathematical simulation studies, e.g., steam quality, injection rate, and amount of steam injected in each cycle.

Field pilot tests show that steam injection is feasible in these oil fields under their respective reservoir conditions. A feasibility study on the development project of the Gaosheng oil field shows that, for such a deep reservoir, steamdrive is feasible by wellbore heat insulation techniques without downhole steam generators. Good prospects are expected, both technically and economically.

Introduction

Most of the reservoirs in the Liaohe area, located in northeastern China, are conventional oil reservoirs. However, some heavy-oil reservoirs were discovered in recent years at a depth greater than 1000 m [3,281 ft] with a maximum of 1700 m [5,577 ft]. Pilot steamsoak tests have been carried out in Gaosheng, Shuguang-I, and Huanxiling oil fields since 1982 to attempt to develop these heavy-oil reservoirs.

There are also some heavy-oil reservoirs in the Shengli oil field in Shandong province. A field pilot test on steamsoak has been carried out since 1994 in the Shanjiasi reservoir, buried at a depth of 1100 to 1300 m [3,609 to 4,625 ft].

Reservoir parameters of the four fields are shown in Table 1. Temperature/viscosity relationships of the crude oils from these reservoirs are plotted in Fig. 1. All these reservoirs are in the lower Tertiary Shahejie formation. They are all characterized by loosely cemented sandstones, gently dipping with very small dip angles, and long productive intervals of multilayered, relatively thick pays having relatively high porosity, permeability, and oil saturation. All these reservoirs are recent discoveries. Wells completed by conventional methods show very low productivity or even no production at all. Steam injection was started at pressures close to the initial reservoir pressure.

Field Pilot Steamsoak

Main oil-producing areas are located in the eastern part of the country, where conventional oil reservoirs are predominant. The heavy-oil deposits discovered so far are buried at depths greater than 1000 m 13,281 ft], with the deepest at 1800 m [5,905 ft]. The heavy-oil reserves have increased recently and the potential of these reservoirs is great. Therefore, pilot tests on steam injection in deep wells have attracted great attention.

According to the requirements of steam injection in deep wells, in-house research and technical preparations for field pilots have been carried out. Steam generators with 16x 10(6) Pa [160 atm] from U.S. companies (working pressure of 18×10(6) Pa [182 atm] and a capacity of 538 × 10(3) or 264×10(3) kJ/h [500,000 or 250,000 Btu/hr]) and wellhead facilities for steam injection were installed: wellbore insulation tubing for deep well steam injection was designed; a feasibility study on deep well steam injection was done; high-temperature downhole packers were developed and installed and high-temperature cement and prestressed casing completion techniques were tested in some deep wells,

Steam huff ‘n’ puff technology was tested for the first time in the Gaosheng oil field in June 1982. Formation damage in this oil field commonly occurred during drilling and completion, resulting in an average well production before steam injection of only 7 to 10 Mg/d [7 to 10 tonnes/D] per well. Thirty-six steamsoak operations in 27 wells were carried out from June 1982 to the end of 1983. The total amount of steam injected was 43 355 Mg [43,355 tonnes] and the additional oil produced was 35 835 Mg [35,835 tonnes]. The oil/steam ratio (OSR) was 0.83 Mg/Mg [0.83 tonne/ tonne]. After steam injection, average daily production for the first month increased from 7 to 10 Mg/d to 20 to 40 Mg/d [7 to 10 to 20 to 40 tonnes/D]. Generally, 20 to 60% steam was injected at a rate of 100 to 240 Mg/d [100 to 240 tonnes/D] per well with a wellhead injection pressure of 13 × 10(6) to 15 × 10(6) Pa [130 to 150 atm] and a reservoir pressure of 12 × 10(6) to 14 × 10(6) Pa [120 to 140 atm]. Each well was equipped with Type 801–1 insulated tubing string and a high-temperature downhole packer set at 1560 to 1630 m [5,118 to 5,348 ft]. Mathematical simulations and actual measurements showed that for most of the wells, the steam quality at the well bottom is 0 %, i. e., hot-water soaking. Significant stimu-lation responses were observed even in such conditions. For example, 2580 Mg [2,850 tonnes] of 41% steam was injected at a rate of 204 Mg/d [204 tonnes/D] into Well 1506 with a pay-zone thickness of 64.8 m [213 ft] at a reservoir pressure of 15 × 10(6) Pa [146 atm] and downhole viscosity of 1500 mPa.s [1500 cp]. After injection, the well was shut in for 3 days, and then opened for production. The well flowed at a rate of more than 110 Mg/d [11tonnes/D] in the first month (see Fig. 2). This well is producing from an interval 200 m [656 ft] from the gas cap, and its production before injection was only 3 Mg/d [3 tonnes/D]. After elimination of formation damage by steam injection, the well production increased dramatically under the action of gas cap drive. Another example is Well 756 with a net pay of 67 m [220 ft] and a daily production of 7.6 Mg/d [7.6 tonnes/D] before 2100 Mg 12,100 tonnes] of 58% steam was injected. This well was shut in for 5 days and then opened to production.

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