No abstract

Thinly laminated and silty deep-water reservoirs of offshore Malaysia have historically posed difficulties in formation evaluation due to complex log responses causing uncertainties in key petrophysical properties like porosity, water saturation, net pay and productivity. Moreover, compartmentalization of the reservoirs due to extensive faulting in this area increases the evaluation challenges. Generally, thinly laminated reservoirs are evaluated either by high-resolution methods, including borehole imaging and whole core analysis; or bulk volumetric approaches, which utilize nuclear magnetic resonance (NMR) and suitable shaly sand saturation equations. Adding silt as an additional component requires a cautious combination of these two approaches. Furthermore, linking the petrophysical evaluation with depositional processes and structural settings using borehole image, acoustic log and formation pressure is key to the future development of the field. Lastly, securing clean formation fluid samples is crucial to design the production strategy. Aforesaid complete dataset was acquired in a deep-water well of offshore Malaysia to assess hydrocarbon potential. While relatively higher resistivity distinguished potential hydrocarbon bearing zones, NMR-based irreducible water saturation was a crucial indicator of possible water-free hydrocarbon production from the silty zones with high water content. Net sand was accurately calculated from the high-resolution borehole image and compared with the standard petrophysical approach. Then, a detail analysis of formation dip, facies and paleo-current direction was performed on borehole image to recognize different depositional processes and structural settings. Formation pressure data was collected extensively to understand reservoir compartmentalization. While the testing zones were selected based on higher free fluid and higher resistivity anisotropy; the precise testing depths on sand laminae were guided by high-resolution borehole image. Later, low contamination downhole fluid samples were collected using focused sampling technique. 2D NMR method and real-time downhole analysis of optical absorbance, refractive indices, fluorescence, density, viscosity and sound speed were used to differentiate formation fluid from the OBM filtrate. The reservoirs were then evaluated integrating the petrophysical properties with the depositional process and structural settings to understand their long-term production potential. This paper represented a case study of an integrated workflow of optimum data acquisition and evaluation of the thinly laminated sand-silt-clay sequence of deep-water reservoirs of offshore Malaysia. Effective and optimum integration of NMR, high-resolution borehole images, formation testing and sampling data provides the robust framework of this formation evaluation workflow to solve the complex petrophysical and geological uncertainties of these reservoirs.

A common operation performed in oilfields is that of plug and abandonment (P&A) whereby a well is permanently closed at the end of its economic life. The regulatory requirements for this operation may vary between countries, however, it is imperative to establish that there is no potential for uncontrolled fluid flow to surface and freshwater aquifers are hydraulically isolated in the wellbore. This entails an accurate assessment of casing and cement integrity including an unambiguous determination of the top of cement (TOC). Towards the end of the plugging operation, a cut is placed above TOC, and the casing is retrieved from the well. There are several challenges associated with cement evaluation in old wells. The well may have gas or gasified fluid in the wellbore, depleted formations making it difficult or impossible to fill the wellbore with liquid prior to cement evaluation logging, casing damage or open perforations may preclude application of pressure to differentiate free pipe from micro-annulus conditions and records on cement and operations history may be sparse. Drilling muds are often weighted using barite or other weighting materials. With an incomplete removal of mud, during well completion, these heavy materials sag, settle and solidify, behind casing, over the life of the well. An often-encountered consequence of solidified mud is to shift casing free-point to a depth well above TOC. Traditional cement evaluation tools find it difficult to identify this condition leading to unsuccessful casing cut and retrieval attempts, and associated time and cost overruns. A new cement logging technique uses electro-magnetic transducers and multiple wave modes to overcome operational and evaluation challenges met in P&A operations. We will present a case study, from Southeast Asia, where the operator was concerned about risks associated with solids sag, above cement, in a 60 years old well. The new service provided accurate cement evaluation across depleted reservoir sections and over intervals with micro-annulus condition. Top of cement was clearly differentiated from the top of solid mud behind casing. A free-point depth was selected and casing was cut and pulled in first attempt. Operator estimated an overall saving of two days in rig time.

A field case study is used to illustrate the hydrocarbon quantification using 2D NMR to distinguish light oil and gas in the challenging synthetic-oil-based mud (SOBM) filtrate environment. The challenge is due to the low-hydrogen index of the gas and SOBM filtrate invasion, which displaces gas from the volume measured by the NMR tool. In scenarios with light oil, the detection is even more difficult because light oil has properties similar to the SOBM filtrate.The required measurement sensitivity can be achieved by acquiring the NMR data in two passes: one with parameters sensitive to gas detection and the other with parameters sensitive to liquid-phase light hydrocarbons. Using data from this case study, we show how using the diffusivity versus T 2int 2D NMR images significantly improve the NMR sensitivity in detecting small formation oil signals separately from the SOBM mud filtrate and how use of the ratio RϭT 1 /T 2app versus T 2app 2D method identifies the gas. The technique utilizes the diffusivity and R contrast between different phases. Because the technique involves both the relaxation and diffusion contrasts, the sensitivity and robustness for hydrocarbon quantification is significantly improved compared with the earlier 1D NMR hydrocarbon-detection techniques.This well was drilled with the expectation of gas; however, initial data suggested the presence of oil which implies a change in completion plans. The acquisition technique provided the information necessary to differentiate the two phases in the SOBM-invaded zone with high confidence and consequence modification of the completion plans.

Acquiring acoustic slowness data in open & cased hole and a reliable cement bond log in one run without jeopardising data quality or increasing rig time is desired for fast and optimize data acquisition. This paper reviews the steps taken to ensure acoustic slowness and cement bond data acquisition fulfils the objective, while minimising the cost in an offshore challenging environment for formations with variable acoustic velocities that could be masked by strong casing arrivals. Crossed dipole acoustic logging is typically preferred to acquire within open hole environment for best quality signal. However, due to drilling challenges this could not be done in the subject well. Data was acquired in 6in open hole and 7" liner (8.5 in Open hole behind) cased hole section together in one run. Shear slowness in slow formation requires propagation of the low frequency dipole flexural wave whereas compressional slowness acquisition and cement bond evaluation requires high frequency monopole data. An improved understanding of cased-hole acoustic modes allowed developing the ability to transmit acoustic energies at optimal frequencies in order to acquire formation slowness concurrently with cement bond. Acoustic data quality in cased hole is dependent on cement bond quality. Poor bonding or presence of fluid between casing and the formation inserts noise in the data by damping the acoustic signal. Hence, understanding of the cement bond quality is critical in interpreting the cased hole acoustic data. The low amplitude of the compressional first arrival indicated the presence of cement bonded with the casing. Absence of casing ringing signal at the beginning and presence of strong formation signal in the VDL indicated good bonding of cement with formation. Filtration of the cased hole acquired semblances were necessary to remove the casing and fluids noises. Acquired data shows good coherency and continuous compressional and shear slowness's were extracted from the good quality semblances. This integrated strategy to acquire the formation slowness and to evaluate the cement bond quality and top of cement allowed meeting all objectives with one tool in single run. The risk of casing waves that could have masked the formation slowness signal was mitigated by transmitting acoustic energies at optimal frequencies with wider bandwidth followed by the semblance processing. The effects of borehole ovality, tool centralization, or casing centralization on waveform propagation were studied to supplement the interpretation. The first times strategic logging application in PETRONAS allowed time and cost saving and fulfilled all data acquisition plan. Data quality assurance and decision tree allowed drafting a workflow to assure data quality. This solution showed importance of smart planning to maximise advanced tools capabilities to acquire acoustic slowness data and cement evaluation in single run in offshore challenging environment.