Abstract

The Chirag Field, located offshore in the Caspian Sea of Azerbaijan, uses permanent downhole gauges to record continuous pressure and temperature in the active producers. Bottom-hole pressure data is used extensively to understand interwell communication and reservoir properties, but bottom-hole temperature data had seen little use. However, we now find that flowing bottom-hole temperature detects interwell communication - with interference delay times consistent with pressure transient analysis - and can be used to estimate interwell permeability. To explain our observations we propose that FBHT responds to the impact of pressure changes. The principal result is a change in the producing GOR, which in turn depends on the speed and magnitude at which a pressure change is transmitted through the reservoir. The effect is pronounced when flowing pressures are below bubble-point and compounded by Chirag's steeply dipping reservoir having gradients versus depth in saturation, temperature, bubble-point, and solution-GOR. Field examples highlight the strong cause-effect relationship between producer-injector and producer-producer pairs, giving evidence of a new interference testing method with wide potential application.

Chirag Field

The Chirag Field in Azerbaijan (Fig-1 and 2) is located offshore in the Caspian Sea. It is part of the Azeri-Chirag-Guneshli (ACG) development and the principal production zone is the Middle Pliocene Pereriv sand. There are currently ten producers and six peripheral water injectors active in Chirag. Production, exported through the Baku-Tblisi-Ceyhan (BTC) pipeline, is 140 Mstb/D with 900 GOR (scf/stb) and less than 1% watercut; water injection is 140 Mbw/D. The structure is an elongated anticline with dips of up to 45 degrees, an overall hydrocarbon column height of up to 1000 meters, and an average stratigraphic thickness of 130 meters in the Pereriv. The two most permeable intervals, the Pereriv-B and Pereriv-D, have a total thickness of 80 meters with 20% porosity and 200 md permeability.

Introduction

The need to monitor, understand, and model temperature and its impact is important across all engineering and geologic disciplines in a producing field. The particular area wellbore temperature sensing is very importance as both early1–6 and recent work7,8 have showed its value. However, such wellbore temperature monitoring and interpretation has focused on the immediate region of the well. At Chirag we find that temperature data also contains interwell information.

Chirag uses permanent downhole gauges (PDHG) to record pressure and temperature in the active producers. The bottom-hole pressure (BHP) data is used extensively to understand interwell communication and reservoir properties, but bottom-hole temperature (BHT) data had seen little use. However, we now find that flowing BHT (FBHT) detects interwell communication, with interference delay times consistent with pressure transient analysis (PTA), i.e., it can be used to estimate interwell permeability. The FBHT data also signals if a voidage change occurs from a location updip or downdip of the producer, as increased support downdip of a producer causes FBHT to increase while increased support updip of a producer (or from the opposite flank) causes FBHT to decrease.

To explain our observations we propose that FBHT responds to the impact of pressure changes. The principal result is a change in the producing GOR, which in turn depends on the speed and magnitude at which a pressure change is transmitted through the reservoir, in addition to the reservoir pressure, FBHP, fluid saturations and properties, and drainage region. The effect is pronounced when FBHP is below bubble-point (the majority of Chirag's producers) and compounded by Chirag's steeply dipping reservoir having gradients versus depth in saturation, temperature, bubble-point, and solution-GOR.

In such cases, a well's FBHT will increase (and GOR decrease) when downdip pressure support increases, i.e., the well's drainage area and streamlines skew downdip toward lower gas-saturations, lower solution-GOR's, and higher temperatures. Conversely, a well's FBHT will decrease (and GOR increase) when updip pressure support increases, i.e., the well's drainage area and streamlines skew updip toward higher gas-saturations, higher solution-GOR's, and lower temperatures. Several examples highlight our observations and estimates are provided for contributing factors.

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