Rubble Zone Below Salt: Identification and Best Drilling Practices

Abstract

A joint-industry project (JIP) from 2005 through 2009 involved acquiring, analyzing, and documenting more than 100 subsalt wells in the Gulf of Mexico (GOM), from shelf to deepwater. The result of this unique study culminated in the compilation of a large and comprehensive database of pertinent petrophysical, geophysical, and drilling data. This database was used to develop a set of suggested best practices for pore-pressure estimation below and around salt bodies. An analysis of the wells in the subsalt JIP indicated that the majority (64%) of the wells studied demonstrated evidence of a subsalt gouge or rubble zone. Sufficient data for this analysis was obtained from 87 JIP wells. Statistical analysis of this large database allowed a wide range of correlations to be established for the subsalt rubble-zone occurrence with salt body structural and geological features. An early diagnostic of rubble zones using logging while drilling (LWD) tools can trigger certain drilling practices to be employed. This paper presents the diagnostic criteria for a subsalt rubble zone and several other confirmatory criteria. The diagnostic identification for a subsalt rubble zone is small-scale discordance between the gamma ray and the resistivity log in the interval immediately below the salt. The relationship between rubble-zone occurrence and salinity changes is discussed.Drilling subsalt rubble zones can be challenging. This paper documents current industry methods for drilling subsalt wells based on the analysis of multiple sources of data. In addition to the subsalt database, data was compiled from an industry questionnaire designed to capture current practices for drilling subsalt wells, a survey and reviews on subsalt drilling-related published literature, and personal interviews with industry professionals. This paper focuses on the following key subsalt (with and without a rubble zone) drilling issues of interest to wellconstruction professionals: the drilling window, casing setting, loss of circulation, wellbore stability, mud-weight scenarios, and leakoff tests. Highlights of each of the aforementioned drilling issues are presented along with well examples.

Allochthonous salt initiation and advance in the northern Flinders and eastern Willouran ranges, South Australia: Using outcrops to test subsurface-based models from the northern Gulf of Mexico

“…A number of theories have been proposed to explain these observations: (1) subsalt basalt shear, in which the advance of salt is accommodated, at least in part, by shearing and thinning of subsalt strata to form a subsalt shear zone (Harrison and Patton, 1995;Harrison et al, 2004); (2) imbricate thrusting to form an antiformal stack beneath salt that has already been emplaced (Jackson and Hudec, 2004;Alexander et al, 2004); (3) imbricate thrust faults ahead of the salt tip that are overridden when the salt advances on one of the more hinterland thrusts Jackson, 2006, 2009); and (4) debris-flow deposits, in which carapace slumps from the frontal scarp and is subsequently overridden by the salt sheet (McGuinness and Hossack, 1993;Kilby et al, 2008;Hudec and Jackson, 2009). However, subsalt rubble zones are not ubiquitous; some wells exit the base of salt sheets into undeformed minibasin strata, and others encounter coherent upturned or overturned strata (Kilby et al, 2008;Saleh et al, 2013).…”

Resistivity-Based Pore-Pressure Prediction—Pitfalls and Solutions

“…The range is from 300 ft to 2,900 ft above the salt and from 300 ft to 2,500 ft below the salt with one apparent anomalous outlier of a 6,200-ft salinity-response zone in one well that tracked closely along the face of a vertical salt dome. These values for the thickness of the salinity response are far thicker than for the disturbed zone (gouge or rubble) below the salt (Saleh et al 2013). Salinity responses occur about the same frequency above and below salt masses.…”

Avaliação Geomecânica De Zonas De Instabilidade Durante a Perfuração De Poços De Petróleo Abaixo De Rochas Evaporíticas

“…Problemas operacionais comumente relatados durante a perfuração de poços nestas zonas estão relacionados a aprisionamento da coluna de perfuração, elevados níveis de arraste e torque da broca, instabilidade da parede do poço, perdas de circulação e influxo de fluidos (6,7).…”

“…Este tipo de região pode favorecer a instabilidade da parede do poço. Durante a intervenção destas zonas a magnitude das tensões circunferenciais é majorada, uma vez que a baixa tensão horizontal combinada com o peso do fluido inadequado pode abrir as fraturas existentes, como ilustra a Figura 2.5, induzindo perdas de circulação de fluidos (6). Esta situação pode elevar significativamente o tempo não produtivo de sonda devido a paralisações da operação para executar procedimentos de controle de poço.…”

“…Esse modelo é derivado do círculo de Mohr e descreve todos os estados de tensão nos quais haja ruptura da rocha por cisalhamento causado pela compressão. A envoltória de ruptura é definida por uma reta tangente aos círculos de Mohr, cuja equação em termos da tensão normal (σ) e de cisalhamento (τ ) é dado pela expressão τ = c + tg (φ)σ, (4)(5)(6) onde c é a coesão do material e φ o ângulo de atrito interno. Os valores dos parâmetros utilizados neste trabalho foram interpretados em ensaios triaxiais em amostras obtidas de testemunho de rochas com idade geológica correlata às formações abaixo do sal do modelo e estão detalhados na tabela abaixo.…”