Geomechanics Archives

Category: Geomechanics

June18

Revealing the impact of lateral changes in rock properties using wellbore stability modelling: “Case study Middle East in Tight Limestone”.

Submitted: ADIPEC , 2021

Maintaining a precise mud weight window when drilling operations encounter only small differences between wellbore collapse and lost circulation can be a demanding task in heterogeneous environments, especially in unconventional formations such as tight limestones. Therefore, understanding the effects of variability in rock properties to minimize wellbore instability in real time and improve borehole condition is crucial in drilling applications.

This paper is focused on the influence of rock heterogeneity in borehole stability and the impact it is expected to have on the safe mud window. The rock mechanical properties (Young’s modulus, Poisson’s ratio and UCS) were estimated using internal client’s correlation based on triaxial and uniaxial tests. In addition, a sensitivity and uncertainty analysis was carried out to identify the range of accuracy required for the input parameters used and how to improve the accuracy of borehole stability analysis.

The investigation involved data acquired from two wells drilled in Saudi Arabia, it shows the strength of the rock will be highly affected by the amount of weak material (siltstone). So due to siltstones contain a significant clay fraction, the gamma ray log was a good indicator of potential weak intervals. For this area 30 API can be taken as a cut off reference, which means that above this number the borehole wall will start to fail. The UCS change significantly in the lateral section affecting the collapse gradient and the mud weight window, meanwhile the friction angle and Poisson’s ratio has a small influence on the result. The stochastic analysis is presented through probabilistic graphs, which enables better decision making.

July11

Geomechanicsby admin

An Integrated Geomechanics and Real Time Pore Pressure Approach helps to Successfully Drill the First Horizontal Well Along the Minimum Horizontal Stress Direction in Tight Sandstone Formations.

Submitted: Paper presented at the Abu Dhabi International Petroleum Exhibition & Conference, Abu Dhabi, UAE, November 2016.

Authors: Vladimir Vasquez, Stephane Le Roux, Julien Heylen, Hassan Eissa, Akram Almazaqei

Paper number: SPE-182977-MS

An operator in the Middle East is seeking to develop its natural gas resources to meet rising energy demand. This operator is drilling horizontal wells along the minimum horizontal stress direction in an attempt to generate multiple transverse fractures during stimulation to increase gas production in unconventional reservoirs.

However, drilling operations in these wells is extremely difficult because of the in situ stress environment; these conditions result in a significantly increased number of stuck pipe incidents, wellbore collapse, incurring remarkably high nonproductive time.

This paper describes the methodology used to define and calibrate the safe mud window for the curve and lateral section of the first horizontal well drilled in tight sandstone formation by conducting a wellbore stability study and real time drilling geomechanics and pore pressure monitoring.

The wellbore stability study incorporates all logs and rock mechanical properties propagated on the direction survey and proposed formation tops of the planned horizontal well X-56. The well was drilled successfully with a mud weight of 95 pcf to 98 pcf in the curve section and 100 pcf to 106 pcf in the horizontal section. Major technical contributions of this paper include: (1) Characterization of geopressures and stress field in different formations calibrated with real time data, (2) back analysis methodology to estimate fracture gradient from formation integrity test (FIT), in addition to this (3) pore pressure zones designation along the stratigraphic column.

July11

Geomechanicsby admin

Developing a mechanical earth model using tectonic deformation coefficients in Ceuta VLG-3676 field, Maracaibo basin, Venezuela.

Submitted: Paper presented at the Offshore Technology Conference, May 5–8, 2014

Authors: Vladimir Vasquez, Carlos Lobo, Axel-Pierre Bois, Hugo Boutin

Paper Number: OTC-25240-MS

Reservoir Eocene C/VLG-3676 of Ceuta field, one of the major reservoirs in Maracaibo lake basin, is characterized by operational challenges that prevent its optimal production. Drilling is challenging and requires large mud-weights to reach target.

In order to optimize drilling and completion operations, a geomechanical study was launched to develop a Mechanical Earth Model (MEM) and use it to improve well trajectory, mud-weight programs, and perforation strategies. The methodology followed when developing this MEM was: data audit and well selection; tectonics and structural-setting analysis; drilling-event analysis; mechanical-stratigraphy definition; rock-property evaluation; pore-pressure estimation; in-situ stress determination; wellbore-stability evaluation; mud-weight window determination; and optimal well trajectory computation.

The three most interesting features of this paper are: the analysis of more than 200 tests performed in laboratory on Eocene “C” cores that shows a very high degree of damage of the cored material; a new in-situ stress field determination method based on tectonic deformation theory; and a stress regime that changes from normal in the Miocene at shallow depths (200–5000′), to strike-slip and reverse in the Eocene at depths greater than 10,000′. This last result is of great importance for producing reservoirs in this area, as it not only impacts the evaluation of optimum mud-weight windows, but also well trajectories and perforation strategies.

July11

Geomechanicsby admin

Importancia de conocer la litología y tipo de formación para diseñar y/o pruebas leak off y leak off extendida en la cuenca del lago maracaibo.

Submitted: Paper presented at the SPE WVS Second South American Oil and Gas Congress held in Porlamar, Edo. Nueva Esparta, Venezuela, 2014

Authors: Canelón R., Vásquez V., Patiño J., Lobo C., Sánchez E.

Paper Number: SPE-WVS-232

En geomecánica, las pruebas Leak Off (LOT, por sus siglas en inglés) y la Leak Off Extendida (XLOT, por sus siglas en inglés) son utilizadas para determinar la magnitud del esfuerzo principal mínimo que actúa en la roca del subsuelo. En trabajos previos se ha analizado la influencia de algunos aspectos, tales como: condiciones de hoyo, elasticidad de la roca, viscosidad del fluido, penetración y permeabilidad del fluido, tasa de bombeo, fracturas pre-existentes, presión de cierre y canalización por mala cementación, en la respuesta de estas pruebas1. Sin embargo, un estudio enfocado en el impacto de la litología en la respuesta de las LOT no se ha realizado.

Una LOT es esencialmente una medida de la resistencia de la formación y depende considerablemente en la litología de la formación que está siendo analizada. Es por ello, que este trabajo tiene como objetivo analizar varias pruebas de esta naturaleza (LOT y XLOT) realizadas en la cuenca de Maracaibo a diferentes profundidades, litologías (contenidos de lutita y arenisca) y tipos de formaciones: nivel de cohesión de los granos (consolidada y no consolidada) y estudiar el efecto de estas variables en la respuesta del LOT. La metodología utilizada comprende cinco (5) pasos:

Recolectar toda la información de la mayor cantidad de estas pruebas llevadas a cabo en varios campos ubicados en el área occidental de Venezuela,
Realizar control de calidad de las respuestas de las pruebas en base a los parámetros mencionados en el artículo de Postler y que pueden afectar la misma, 2. Caracterizar la litología y el tipo de formación donde se desarrolló la prueba, 3. Analizar e interpretar las respuestas de las pruebas recolectadas, 4. Establecer una relación entre las respuestas y las variables caracterizadas en el paso 2 y 5. Identificar la correspondencia entre los minerales de las arcillas presentes en las formaciones analizadas y la respuesta plástica de las lutitas. Los resultados muestran lo siguiente: pruebas LO (Leak Off) ejecutadas en formaciones no consolidadas y en litologías con alto contenido de lutita (80%) exhiben una respuesta plástica, en contraste, formaciones bien consolidadas con alto contenidode lutita muestran un comportamiento elasto-plástico. Asimismo, se observó que formaciones no consolidadas con alto contenido de arena (80%) también actúan como un material elasto-plástico. Finalmente, una roca bien consolidada con alto contenido de arena (80%) presentó una respuesta elasto-plástica en la prueba. Los resultados mostrados corroboran el impacto de la litología y tipo de formación en las respuestas de las pruebas LOT y XLOT que deben ser tomadas en cuenta a la hora de la interpretación de dichas pruebas. De acuerdo a lo anteriormente expuesto, se concluye que en formaciones no consolidadas, el comportamiento plástico o elástico está gobernado por la litología de la formación probada (contenido de lutita y arena); por su parte, una formación consolidada se comportará como un material elastoplástico independientemente del porcentaje de lutita del intervalo de la formación analizada.

July11

Geomechanicsby admin

Drilling in Pakistan: Predrill wellbore stability study in a highly anisotropic rock formation for complex conditions in an exploration well.

Submitted: ADIPEC, 2021

Authors: Vladimir Vasquez, Omer Mohammed

Numerous wellbore instability problems were encountered while drilling through a shale formation in Pakistan gas fields. These problems become greater as the well inclination increases. Consequently, a wellbore stability study in the exploration well X1 (ST) was conducted to address those complications. Because of a lack of published studies regarding wellbore instability problems in this area, this valuable case study should serve as an example for similar fields in that region.The paper provides a detailed description and well bore stability analysis which include drilling issues encountered in the offset wells, failure analysis and geopressure evaluation. Also, it presents a strategy to maintain mechanical stability in shales. Finally, it carried out an uncertainty analysis using Monte Carlo simulations for quantifying uncertainty in model outputs and obtaining the required mud weight to drill successfully, which form part of the strategy for designing an optimal drilling fluid program.

The drilling events and data revealed potential anisotropic strength effects caused by bedding planes and abnormal pore pressure in this shale formation. The 1D wellbore stability model showed shear failure in rock exhibiting high stress concentration, which means that the mud weight should have been higher than that used to prevent borehole collapse. FMI data were used to estimate the angle of attack estimated in the X1 (ST) well, and revealed several intervals in the critical area that were characterized by low strength. The workflow proposed makes up a complete study useful for developing well bore stability analyses which optimize decision making in drilling process design.

The technical contributions of this study is that the conventional model prediction gives one safe mud weight for specific input data while the uncertainty model prediction provides a confidence level to each calculated safe mud weight range. This paper clarifies how to apply this procedure. Features like evaluation of attack angle and integration of uncertainty and sensitivity analysis are novel components for completing well bore stability modeling in exploration areas.