API RP 44: Second Edition
SDO: API: American Petroleum Institute
DOD Adopted ANSI Approved Approved
Proper management of production from a natural gas or petroleum reservoir can maximize the recovery of the hydrocarbon fluids (gas and oil) originally in the reservoir. Developing proper management strategies requires accurate knowledge of the characteristics of the reservoir fluid. Practices are recommended herein for obtaining samples of the reservoir fluid, from which the pertinent properties can be determined by subsequent laboratory tests.
The objective of reservoir fluid sampling is to collect a sample that is representative of the fluid present in the reservoir at the time of sampling. If the sampling procedure is incorrect or if samples are collected from an improperly "conditioned" well, the resulting samples may not be representative of the reservoir fluid. A non-representative sample may not exhibit the same properties as the reservoir fluid. The use of fluid property data obtained from non-representative samples, however accurate the laboratory test methods, may result in errors in reservoir management. Poor planning can also result in incomplete data being taken during the sampling program. Incomplete data can make it difficult or impossible for laboratory personnel to perform and interpret tests that provide accurate and meaningful fluid property information.
Reservoir fluids found in gas and oil fields around the world vary greatly in composition. In some fields, the fluid is in the gaseous state and in others it is in the liquid state; frequently, gas and liquid coexist in a given reservoir. The rocks which contain these reservoir fluids also vary considerably in composition, and in physical and flow properties. In certain cases, this can serve to complicate the sampling procedure. Other factors, such as producing area, height of the column of hydrocarbon fluid, fracturing or faulting, and water production also serve to distinguish one reservoir from another. The combination of all these factors affects the choice of sampling methods and preparations for sampling.
When a reservoir is relatively small, a properly taken sample from a single well can be representative of the fluid throughout the entire reservoir. For reservoirs which are large or complex, samples from several wells and/or depths may be required. Significant variations in fluid composition often occur in very thick formations, in really large reservoirs, or in reservoirs subjected to recent tectonic disturbances. Additional sampling during the later life of a reservoir is not uncommon because production experience can show that the reservoir is more complex than earlier information indicated.
Methods for sampling reservoir fluids fall into two general categories. They are referred to as subsurface sampling or surface sampling and, as the names imply, each category reflects the location at which the sampling process occurs. Subsurface sampling may also be referred to as downhole or bottomhole sampling. Modern open-hole wireline formation testers now provide the means of recovering representative samples before the effects of subsequent production take place. Selection of one particular method over another is influenced by the type of reservoir fluid, the producing characteristics and mechanical condition of the well, the design and mechanical condition of the surface producing equipment, the relative expense of the various methods, and safety considerations. Detailed descriptions of recommended sampling methods are presented in subsequent sections of this document. Factors which should be considered in choosing a method are also discussed.
The choice of either the surface or downhole sampling method cannot be considered a simple or routine matter. Each reservoir usually presents certain constraints or circumstances peculiar to it. For example, field operation requirements can impose restrictions on the preparation and execution of a sampling program; sand production or downhole equipment in the well may limit the use of some of the equipment normally used in the sampling operation. Wells that exhibit rapid variations in production rate present special problems in making the necessary measurements with acceptable accuracy. Seasonal or daily weather changes can also influence the sampling operation. Thus, the details of a given sampling procedure often require modification to circumvent local problems. These modifications are usually made based upon on-the-spot judgments. Some of the modifications which can be made to accommodate special situations are presented herein.
Conditioning a well before sampling is almost always necessary. Initial well testing or normal production operations often result in the fluid near the wellbore having a composition which has been altered from that of the original reservoir fluid (for reasons described later). The objective of conditioning the well is to remove this altered (non-representative) fluid. Well conditioning consists of producing the well at a rate which will move the altered fluid into the wellbore and allow it to be replaced by unaltered (representative) fluid flowing in from further out in the reservoir. Well conditioning is especially important when the reservoir fluid is at or near its saturation pressure at the prevailing reservoir conditions because reduction in pressure near the wellbore, which inevitably occurs from producing the well, will alter the composition of the fluid flowing into the wellbore. The types of problems which can be encountered in conditioning a well, and methods for their control are discussed.
Forms for recording the data obtained during well conditioning and sampling operations are included in this document. These data furnish a history of the sampling operation, provide complete identification of the samples, and aid in interpreting the results of the laboratory examination of the samples. The use of these forms (or equivalent computer based information) encourages the collection and reporting of all essential data.
The development of modern electronic flow metering allows flow rate data to be collected and recorded very rapidly in "real time." This includes field calculations and data recording in spreadsheet form, which can now be done with hand-held, laptop, or notebook-type computers. These results can be transmitted electronically (e-mail) or diskettes can be mailed in addition to or instead of hardcopy forms. This type of computer-based data collection will be preferred by many operators.
The use of modern electronic flow metering and computer equipment does not mean that wells can be conditioned any more quickly or that gas and liquid flow rate data will automatically become more representative of reservoir fluid. However, use of such equipment and methods, if properly calibrated and understood, should reduce errors and generally improve the quality of data collected.
Personnel who are inexperienced in reservoir fluid sampling are strongly advised to consult with experienced personnel regarding the planning of a sampling program, particularly if complicating situations are anticipated.
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