Under the general designation of thinly bedded reservoirs, a large class of hydrocarbon bearing formations exhibit widely varying characteristics. The thickness of the laminations may be as little as few millimeters and as large as several centimeters. While laminated reservoirs are generally assumed to be made up of sand-shale sequences, many include carbonate layers or more complex lithology.
In the case of thick laminations, formation evaluation is performed based on deconvolution guided by microresistivity images. The method is shown to yield credible answers in a Gulf of Mexico example. In the more common case of very fine laminations, hydrocarbon saturation is computed using electrical anisotropy in conjunction with advanced petrophysical interpretation. The overall method includes therefore two important sequential steps. The first step consists of accurately determining Rv and Rh, i.e. perpendicular and parallel resistivities. The second step consists of interpreting these Rv and Rh in terms of Vshale and hydrocarbon content.
The determination of Rv and Rh is accomplished in wireline logging via the new Xaminer™ Multi-Component-Induction sensor, capable of performing the measurement at any relative angle between the well axis and the laminations bedding. In LWD, the measurement of Rv and Rh can be easily performed in high angle wells when the relative dip is superior to 45 degrees. The sensor is a tilted coil array, widely available named ADR™ Azimuthal Deep Resistivity Sensor.
The derivation of hydrocarbon saturation from Rv and Rh can be accomplished by means of the interpretation package LASSI; its algorithms are based on the work of Jim Klein, David Allen, Rick Mollison, John Quirein and Jean-Baptiste Clavaud, and require the knowledge of the resistivity of the shale laminations and yield estimation of hydrocarbon content and of Vshale,
Complementing the resistivity anisotropy approach in laminated reservoirs, magnetic resonance and fluid sampling help further identify the formation fluids. Magnetic resonance logs are able to differentiate between water, oil, and gas. In most instances it is possible to recognize reservoir hydrocarbon from oil base mud through advanced techniques. A more direct method consists of drawing fluid samples from the laminated reservoir with wireline or LWD formation testers and bringing them to the surface for analysis. In laminated formations, oval pads and straddle packers are particularly well suited for maximizing the chances of success.
About Roland Chemali
Chief Petrophysicist – Sperry Drilling
Roland Chemali is Chief Petrophysicist for Sperry Drilling Services and Halliburton Technology Fellow. Distinguished Speaker SPWLA 2004 and 2008, and Distinguished Lecturer SPE for 2010-2011. Roland earned engineering degrees from the Ecole Polytechnique of Paris and from the French Petroleum Institute and a Master’s in Mathematics from LSU. Roland has co-authored over 60 papers and patents in formation evaluation and geosteering. He has received multiple awards including the Technical Achievement Award from the Society of Petrophysicists and Well Log Analysts (SPWLA). He currently serves as President of SPWLA 2012-13