Modified pulse sequence to estimate properties

Abstract

Methods and related systems are described for estimating fluid or rock properties from NMR measurements. A modified pulse sequence is provided that can directly provide moments of relaxation-time or diffusion distributions. This pulse sequence can be adapted to the desired moment of relaxation-time or diffusion coefficient. The data from this pulse sequence provides direct estimates of fluid properties such as average chain length and viscosity of a hydrocarbon. In comparison to the uniformly-spaced pulse sequence, these pulse sequences are faster and have a lower error bar in computing the fluid properties.

What is claimed is: 
     
         1 . A method of estimating a property of a material, comprising:
 inducing a change in the material comprising a modified pulse sequence;   obtaining a measurement of a response to the inducing, wherein the response comprises exponential decay following the inducing;   taking weighted linear combinations of the measurements to determine moments of a variable from the measured exponentially decaying data; and   determining a property from the moments.   
     
     
         2 . The method of  claim 1 , wherein a pulse sequence is designed in time domain according to a formula determined by the property being estimated. 
     
     
         3 . The method of  claim 1 , wherein a pulse sequence in frequency domain is designed according to a formula determined by the property being estimated. 
     
     
         4 . The method of  claim 1 , wherein a pulse sequence is non-uniform in time in both linear and logarithmic scales. 
     
     
         5 . The method of  claim 1 , wherein a pulse sequence in time is predetermined using a power law. 
     
     
         6 . The method of  claim 1 , wherein a pulse sequence in frequency domain follows is predetermined using a power law. 
     
     
         7 . The method of  claim 1 , wherein a pulse sequence in time is selected for the property to be directly computed from the measurements. 
     
     
         8 . The method of  claim 1 , wherein a pulse sequence in a frequency domain is selected for the property to be directly computed from the measurements. 
     
     
         9 . The method of  claim 1 , wherein the weights in the weighted linear combination depend on the moments being estimated. 
     
     
         10 . The method of  claim 1 , wherein the properties are used to compute petro-physical parameters. 
     
     
         11 . The method of  claim 1 , wherein the material comprises a subterranean formation. 
     
     
         12 . The method of  claim 1 , wherein the material comprises water or hydrocarbon or both. 
     
     
         13 . The method of  claim 1 , wherein the properties comprise the mean chain length and viscosity. 
     
     
         14 . The method of  claim 1 , further comprising obtaining indications of property changes over temperature and pressure and 
     
     
         15 . The method of  claim 14 , further comprising determining phase transitions within the material from the changes in properties. 
     
     
         16 . The method of  claim 1 , further comprising determining an uncertainty in the properties from an uncertainty of the measurement. 
     
     
         17 . The method of  claim 1 , wherein the obtaining occurs in a subterranean formation traversed by a wellbore. 
     
     
         18 . The method of  claim 1 , wherein the material is a fluid. 
     
     
         19 . The method of  claim 1 , wherein the material is a rock solid and the properties are petrophysical properties of the rock solid. 
     
     
         20 . The method of  claim 1 , wherein the inducing comprises nuclear magnetic resonance 
     
     
         21 . The method of  claim 1 , wherein the property is a moment of relaxation-time or a diffusion distribution or a combination thereof. 
     
     
         22 . A system for estimating a property of a material, comprising:
 an inducer that is adapted to induce a measurable change in the material;   a sensor to measure a material response that exponentially decays following an induction;   a controller coupled to the inducer, adapted and programmed to repeat the induction at predetermined intervals; and   a processing system adapted to estimate the property based at least in part on the sensor measurements, wherein the intervals are predetermined comprising the material response.