Single-Phase Bottomhole Sampling Technology
Authors: Jamaluddin A.K.M. , Ross B. , Ross D. , Hashem M. 
Published: March 2010

Oilphase, a Schlumberger company, has developed and successfully tested a novel Single-phase Sampling Technology (SST), which is in use worldwide. In this system, the collected reservoir fluid pressure is maintained at or above reservoir pressure during retrieval by the release of a pre-set nitrogen charge. Subsequent to collecting the single-phase reservoir fluid, experiments were conducted in the laboratory to demonstrate the importance of SST. Results indicate that the onset of asphaltene flocculation pressure indeed varies with the amount of asphaltene content in the reservoir fluid. The collection of a non-representative reservoir fluid sample may carry a non-representative asphaltene amount due to irreversible precipitation, and hence, result in characterization of a non-representative asphaltene flocculation onset. Therefore, asphaltene characterization on a nonrepresentative reservoir fluid sample will lead to either over- or under-design of the production system.

Introduction

Reservoir fluids containing hydrocarbon solids are common in the petroleum industry. The primary constituents of the solids are waxes and asphaltenes. These compounds remain in solution under reservoir temperature and pressure conditions. The hydrocarbon solids will destabilize from the reservoir fluid when the thermodynamic equilibrium conditions (that is, temperature, pressure, and composition) are altered.

Asphaltenes are dark brown to black solid molecules with no definite melting point. They decompose while heating, and leave a carbonaceous residue. They are non-crystalline substances or mixtures of relatively high molecular weight fractions(1) of bitumen with characteristics of strong aromatic polar substances(2, 3). Asphaltenes are defined as the n-heptane insoluble fraction of crude oil(4, 5) that are also soluble in toluene(6). They are classified by the particular solvent used to precipitate them(7).

Various laboratory techniques are used to determine the onset of asphaltene flocculation.(8–13) Most of these techniques are dependent on the oil characteristics. The most important characteristic is the colour of the oil. For dark-coloured oil, the popular light-scattering technique using a laser source in the visible spectrum becomes inadequate. In this case, near-infrared (NIR) light spectrum is necessary to identify any changes in the response due to a phase change phenomenon(14).

Once flocculated, asphaltenes have the propensity to grow in size, and subsequently, may start to deposit on the reservoir and production circuit (Figure 1). Flocculation and deposition of asphaltenes have reportedly caused a multitude of operations problems(15), thereby increasing the cost of production significantly. From a reservoir management point of view, high onset of asphaltene flocculation pressure can severely limit the primary recovery potential. To avoid the reservoir pore throat clogging, operators may need to implement a reservoir pressure maintenance scheme. Recently, Kabir and Jamaluddin(16) have presented their work related to the requirements of pressure maintenance in order to produce from the South Kuwait Marrat formation without having to experience the asphaltene deposition problem.

Regardless of various approaches, we believe that the correct operating procedure to minimize the asphaltene problem is not well understood. This could be attributed to our inability to retrieve a representative bottomhole sample using the conventional bottomhole sampler.