The paper was presented at the SPE/DOE Unconventional Gas Recovery Symposium of the Society of Petroleum Engineers held in Pittsburgh, PA, May 16-18, 1982. The material is subject to correction by the author. Permission to copy is restricted to an abstract of not more than 300 words. Write: 6200 N. Central Expwy., Dallas, TX 75206.

Introduction

Gas hydrates can occur naturally in subsurface formations under equilibrium pressure conditions. These conditions exist in most arctic and deep offshore regions. During exploratory drilling for oil and gas in the arctic, operators have encountered insitu hydrates that pose engineering problems. Although these zones are often non-productive, they must be evaluated for well control purposes and well completion design. The potential of hydrates as an energy resource has also received much attention over the past ten years. Studies of production methods have been undertaken and hydrate coring procedures have been investigated. With the increased interest in naturally occurring hydrates, the need for improved detection and evaluation methods has also increased. In this paper, logging of hydrates is discussed and selected logs from four arctic paper, logging of hydrates is discussed and selected logs from four arctic wells are examined. A new procedure based on temperature log analysis is described. The concept of a downhole heater for use with drill stem testing is also described for testing and evaluation of hydrate intervals.

OPEN-HOLE LOGS FOR HYDRATE DETECTION

Insitu gas hydrates can occur within and below permafrost in arctic regions. Like permafrost, hydrates show a relatively high induction log resistivity and sonic log velocity, as well as over-gauge hole. Sonic logs of hydrate intervals often exhibit cycle-skipping due to drilling muds becoming locally gassy from decomposed hydrate. The spontaneous potential log shows little deflection opposite hydrates, but significant response next to gas and water zones. The mud gas log is a good indicator of hydrate. Generally, a sharp increase in total mud gas occurs as hydrate zones are penetrated during drilling. The responses of the logging tools are significantly affected by the lateral extent of hydrate decomposition which occurs due to heating the formation by the drilling process. Individual open-hole logs can provide useful information on insitu hydrates, but are unlikely to yield definitive evidence unless cross-correlations between logs are used. In this paper, five log correlation techniques are presented for application to hydrates:

  1. Density/neutron porosity crossplot

  2. Apparent resistivity and water saturation

  3. SP/resistivity ratio crossplot

  4. Dual temperature logs and hydrate equilibrium

  5. Temperature difference plot (hydrate thermal conductivity effects).

These correlation methods with example applications are discussed below.

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